Error and Variety Coins

Most collectors and noncollectors who check the coins in their change eventually encounter pieces that look different, for lack of a better term. The coins may be missing details like a Mint mark or part of an inscription. On other coins, some design elements may be blurry or even appear doubled. Other coins may be in a different color than normal, thinner than usual or otherwise deviate from the norm in appearance or physical standards.

Individuals finding such pieces may have found error coins or die varieties, or coins from a later die state or coins representing a die stage. Alternatively, they might have encountered coins that have been damaged or have otherwise changed while in circulation, or that were deliberately altered.

Some error coins and die varieties carry premiums, from a few dollars to thousands of dollars. Many pieces are collectible but carry no premium. Damaged and altered coins are worth face value only, and if too badly damaged, may not even work in vending machines.

Entire books are devoted to errors, varieties, die states and die stages, so this information is best viewed as an introduction to the subject. Collectors wanting more information may want to read the weekly "Collectors' Clearinghouse" column or the twice monthly column "Varieties Notebook" in Coin World, or acquire any of the specialty books on these topics.

A good beginning place is with definitions of the broad terms used in reference to error and variety coins. We also examine most of the categories of errors, varieties, states and stages.

An error coin is one that deviates from the norm because of an accident, mistake or mishap at any stage of the minting processes.

A die variety represents a coin produced by a die that differs – from the moment of the die's production – from all other dies for the same denomination, design type or subtype, date and Mint mark. The definitions for specific kinds of die varieties that follow should make this concept easier to understand. Among the 20th century coins considered "varieties" are such coins as the 1955, 1972 and 1995 Lincoln, Doubled Die Obverse cents. Die varieties are encountered less frequently in the 21st century because of modernization of die production processes.

While some classify die varieties as errors, not all die varieties were produced by mistake. The scarcity of die steel in the late 18th century and early 19th century led Mint officials to practices their 21st century counterparts would reject. For example, a die with one date (1798, for example) might be repunched with another date (1800) and used. (1800/798 Draped Bust cents actually exist!). Such coins are a form of die variety called "overdates," and while it is possible that some overdates occurred by mistake, most of the early overdated dies likely were created deliberately.

Deliberate die varieties were produced as recently as mid 20th century because of the temporary closing of the San Francisco Mint beginning in 1955. Reverse dies stamped with the San Francisco Mint's S Mint mark purposely were restamped with the D Mint mark of the Denver Mint after the California facility was closed. The dies with the over Mint marks were placed into use at the Denver Mint and used to strike a series of 1955-D/S Jefferson 5-cent coins.

A die state simply represents a specific period within a die's use, marked by the presence or absence of wear or abrasion on the die. Dies wear as they strike coins; the wear is evident on the coins they strike.

A die stage represents a period within a die's life marked by something other than wear or abrasion. Many examples will be detailed.

Fewer and fewer

Errors are in very short supply compared to total mintages, especially in the 21st century. U.S. Mint officials have introduced equipment that has reduced the number of errors produced and catches more of those pieces that are made. Because of the reduced numbers of errors being made and released, prices for some 21st century errors can be higher than for older examples of the same type of error.

As with error coinage, the U.S. Mint has introduced technological changes that have made most of the significant forms of varieties impossible (or highly unlikely). Repunched Mint marks and over Mint marks (both defined later) became impossible at the beginning of the 1990s.

A major doubled die (one with the degree of doubling found on the classic varieties listed in Coin World's Coin Values has not been produced since the late 1990s. Changes in the die-production process introduced in 1996 and 1997 eliminated the major doubled dies but still permitted production of minor examples, a Denver Mint technician told a Coin World staff member in 1996. That technician's statement has gained support with the discovery of numerous coins of various denominations, dates and Mints. The degree of doubling present on these coins is minor compared to that on classic varieties such as the 1955 and 1972 Lincoln, Doubled Die Obverse cents.

Another entire category of error began circulating in early 2007 following the release of the first Presidential dollars: errors involving the edge inscriptions.

Values

Values for error coins, die varieties and coins of particular die states and die stages depend on the same factors affecting normal numismatic merchandise: supply, demand, condition and the knowledge levels of the buyer and seller in a particular transaction.

Some die variety coins, such as the 1955 Lincoln, Doubled Die Obverse cent, because of publicity and dealer promotion, "cross over" and become popular with general collectors; thus, demand is higher for a fixed supply, and values are correspondingly higher.

Rare does not necessarily mean great value. Many older error coins struck in small quantities are available for a few dollars. Even errors that are considered unique are often available for several dollars.

The knowledge levels of buyers and sellers can play a role in determining the price (in contrast to market value) of a coin, die variety, die stage or die state. Knowledgeable collectors and specialist dealers generally have a full understanding of the cause of a particular error, variety, state and stage, and know the market for such pieces. They thus can agree on a fair price that likely will be upheld in the market.

Conversely, online auction sites like eBay have opened up new venues for buying and selling coins, where buyers and sellers may lack the same levels of knowledge as the experts, and sometimes buy and sell at prices the established market would not bear. New collectors of these kinds of coins should exercise caution when buying online; consult with knowledgeable specialists before paying for something a seller promotes as the "hot new error!" New sellers should research their finds before selling and make sure they have accurately described the coins they are offering, and they should place starting prices at appropriate levels.

Finding oddities

Error, variety, die state and die stage coins can be found in circulation, unlike many other collector coins. Some collectors go to banks and obtain large quantities of coins to search through; coins not bearing errors are returned to the bank. Many error, variety, die state and die stage coins, particularly of the minor classification, can be discovered simply by going through pocket change. All it takes are sharp eyes and knowledge of what to look for.

Major errors, particularly those that leave the coin misshapen, are increasingly difficult to find. The Mint has improved its ability to keep such pieces from escaping the facility where they were struck, thus making such pieces rarer than before. Collectors and dealers have far fewer opportunities to acquire such pieces than in the past.

Many collectors are adept at what is called "cherrypicking." They use their superior knowledge of die varieties when searching a dealer's stock to obtain scarcer pieces at prices less than what a specialist might charge.

Some of that knowledge comes from a clear understanding of the minting process. The minting of a coin is a manufacturing process and should be fully understood by anyone interested in collecting and studying error coins (see "How coins are made"). Many forms of alteration and damage inflicted outside the Mint resemble certain types of errors, but none precisely duplicates a genuine Mint error. Collectors who understand the minting process should be better able to distinguish between errors, damage and alteration.

The following section classifies the pieces under review into three categories: those involving the die (varieties, states and stages), planchet errors and striking errors.

For those pieces involving the dies, we will list whether they represent a variety, state, stage or something else.

Planchet errors are the result of defective or improperly selected planchets.

Striking errors are coins whose errors result from a mishap occurring during the actual striking process.

The list that follows is not comprehensive. Other forms of errors exist, some rarely encountered, including some requiring additional explanation.

Die varieties, states and stages

Abraded dies: Mint employees use an abrasive on dies to extend their working life and to remove such things as clash marks, die scratches, dirt and grease. If the die is abraded too much, details may be erased or small raised lines may appear on the coins. Most over-abraded errors have little value, but some are exceptions, most prominently the 1937-D Indian Head, Three-Legged Bison 5-cent coin. Regarded by many as a die stage.

BIE: The term commonly used for minor die stage errors affecting the letters of the word LIBERTY on Lincoln cents. A small break in the die between the letters, especially BE, often resembles the letter I, hence the BIE designation. Such die stages are much more common on the coins of the 1950s and early 1960s than on more recent issues. Experienced collectors tend to be less interested in such errors in the early 21st century than they were 30 or 40 years ago. They generally carry little to no premium.

Clashed dies: When, during the striking process, two dies come together without a planchet between them, the dies clash (come into direct contact). Depending on the force with which the dies come together and the nature of the designs, a portion of the obverse design is transferred to the reverse, and a portion of the reverse is transferred to the obverse. Coins struck from the clashed dies will show signs of the transferred designs; the transferred design elements are called clash marks. Although the cause of this type of error occurs during the striking process, the die is affected; thus, it is considered a die stage error. Minor clash marks rate only a slight premium, while stronger clash marks might bring slightly higher premiums. The ultimate clashed dies bear clash marks from different denominations, as with several different 19th century coins; these coins can sell for thousands of dollars.

Design mule: The result of dies for two different design types of the same denomination being used to strike a coin. The only known example on a U.S. coin is a 1959-D Lincoln cent struck with the 1958-style Wheat Heads reverse rather than the 1959-style Lincoln Memorial reverse. The Secret Service has not determined this coin to be counterfeit, although grading services have been reluctant to authenticate the coin and some die variety specialists are skeptical. A convicted and imprisoned counterfeiter-murderer claims to have made this piece, although he has offered no proof of his claims and most are skeptical of his claims. Design mules exist for the coins of other countries.

Die breaks, chips, cracks, gouges, scratches: Dies, like any other piece of steel, are subject to all sorts of damage (resulting in different kinds of related die stages). Any incused mark on the die leaves raised areas on coins. Breaks and cracks are similar, appearing on coins as raised lines. A die break affects a larger area than the die crack, and breaks often result in pieces of the die falling out. A die chip occurs when a small portion of the die breaks away, while gouges and scratches generally occur when a foreign object scores the surface of the die. Minor examples of these carry little to no premiums, although the coins are collectible.

A rim-adjacent die break is often, though somewhat strangely, referred to as a "cud." It occurs when the die breaks at the rim and a piece of the die falls out of the press. The metal of coins struck from that die flows up into the missing area, resulting in a raised blob of metal bearing no image. The side of the coin opposite the raised blob is often weak and indistinct; this is because metal flows along the path of least resistance, flowing into the uncontrolling broken area of the damaged die and not enough into the defined recesses of the opposite, normal die. A retained major die break occurs when the die breaks at the rim, but the piece does not fall out. Coins struck from these dies show the break, but also depict the image inside the break. The design element inside the broken area may be mispositioned slightly because the chunk of broken die shifted. Coins with major die breaks carry premiums, with value dependent on coin, date and size of break.

Double-denomination mule: The result of dies for two different denominations being used to strike a coin. The only known double-denomination U.S. mules were struck from a Washington quarter dollar obverse die and Sacagawea dollar reverse die, on a Sacagawea dollar planchet; from a 1995-D Lincoln cent obverse die and Roosevelt dime reverse die, on a cent planchet; from a 1995 Lincoln cent obverse die and Roosevelt dime reverse die, on a dime planchet; and from a 1999 cent obverse die and dime reverse die, on a cent planchet. They are similar in concept to the design mules (dissimilar dies being used to strike a coin). Authentic U.S. double-denomination mules are extremely expensive coins, bringing five-figure prices.

Doubled dies: If, during the hubbing and die making process, a misalignment between hub and partially completed die occurs, overlapping, multiple images may appear on the die, creating a die variety. Coins struck from the die will show the overlapping images, like the doubled date, IN GOD WE TRUST and LIBERTY on the 1955 Lincoln, Doubled Die Obverse cent. Die doubling, on coins with raised designs, features a rounded second image; on incused designs, the second image is flat and shelflike. At the corners of the overlapping images are distinct "notches" on coins with raised designs. A tripled or quadrupled die is caused by the same misalignment, but bears a tripled or quadrupled image. Major U.S. doubled dies are considered impossible since 1996 to 1997 due to technological improvements at the Mint, but minor examples continue to appear. Traditional doubled dies can only occur when more than one impression of hub into die is required, and the alignment between hub and die shifts or distorts between hubbing operations. Some specialists suspect these occur when a hub and die blank are tilted in relation to each other at the start of the hubbing operation. Doubled die varieties can sell for a few dollars or many thousands of dollars each, based on rarity, degree of doubling and overall popularity of the specific variety.

Engraving varieties: While more common on the dies of the 18th and 19th centuries, engraving varieties have been made on modern dies. On the earlier dies, numerals and letters were often repunched to strengthen the design, punched in upside down or otherwise out of alignment, and sometimes, wrong letters or numbers were punched into the die. On more modern dies, engraving errors include the use of the wrong size Mint mark by mistake and Mint marks placed too close to design elements or otherwise too far from their intended locations. Other "engraving" errors, discussed in separate sections, include doubled dies, overdates and repunched Mint marks. All are die varieties. Premiums range from low to extremely high depending on rarity and popularity of the variety.

Gouges: Die gouges represent damage incurred on a die after it is produced. A tool or other hard object may scrape across the die, leaving a gouge in its surface. The gouge appears raised on the coin. Value depends on the severity and placement of the gouge; some die gouge pieces have been heavily hyped. A gouge is regarded as a die stage.

Misaligned dies: Although one side of the coin appears to have been struck off-center, the planchet was not off-centered during striking, as in an off-center coin. A misaligned die occurs when one die is horizontally displaced to the side, causing only a partial image to appear on that side of the coin. However, unlike the off-center coin that it resembles, only one side is affected. The other side is normal. Some specialists classify misaligned dies as striking errors. Premiums differ, though most are relatively low.

Misplaced dates: A misplaced date is a relatively recently studied die variety. Researchers have discovered numerous coins with numbers from the date punched well away from the region of the die where the date was punched. For example, coins have been found with a number or numbers punched into the dentils along the rim or into a major design element. While theories abound, no one is sure whether this punching was accidental or deliberate. Premiums vary; refer to specialist books.

Missing Mint marks: Some coins that should have Mint marks don't. While in most cases the cause of the missing Mint mark is a filled die (thus temporary, and worth very little), sometimes the Mint mark is not punched into the die, creating a die variety. A series of missing Mint mark errors occurred on coins placed into Proof sets, from 1968 to 1990; all carry substantial premiums. Another valuable example is the 1982 Roosevelt dime without Mint mark. They, too, carry a strong premium. Collectors should remember that many normal, older coins lack Mint marks, as do all Lincoln cents struck at the Philadelphia Mint.

Overdates: When one or more numerals in the date are engraved, punched or hubbed over a different numeral or numerals, both the original date and the second date can be seen. Twentieth century hubbed examples include the 1943/2-P Jefferson 5-cent coin and the 1942/1 Winged Liberty Head dime (both are also doubled dies). The traditional, pre-20th century overdate occurred when one date was punched over another date, as on the 1818/5 Capped Bust quarter dollar. All overdates are die varieties. Premiums vary, with some pieces bringing tens of thousands of dollars, especially in high grade.

Over Mint marks: A form of repunched Mint mark, but when punches of two different Mints are used. Examples include the 1944-D/S Lincoln cent and the 1938-D/S Indian Head 5-cent coin. These can occur accidentally or deliberately, as noted earlier. All are die varieties. Some are inexpensive, while others carry higher premiums.

Repunched Mint marks: Mint marks were punched into each individual working die (for coins issued for circulation) by hand with mallet and punch, before changes were made to the process in 1990 and 1991. Under the old system, several blows to the punch were needed to properly sink the Mint mark into the working die. If the punch was not properly placed after the first blow, a multiple image could result. A coin identified as a 1960-D/D Lincoln cent has a repunched D Mint mark, for example. Coins with RPMs are die varieties. Most RPMs carry low premiums.

Rotated dies: Most U.S. coins have the obverse and reverse sides oriented so each side is upright when rotated on a horizontal axis. The alignment difference between the two is 180 degrees. However, if the dies are aligned at anything other than 180 degrees, the dies are considered rotated. The Mint considers coins rotated by 5 degrees or less within tolerance. Dies can rotate from the optimal position for various reasons although the cause for a specific coin may not be identifiable. Some specialists consider rotated dies to be striking errors since the die’s face is unchanged. Prices vary from no premium to premiums of several hundred dollars.

Worn dies: Dies have a limited life, based on the hardness of the coinage metal being struck and the striking pressures involved. When a die wears beyond a certain point, details around the rim tend to flow into the rim, while other details weaken. The surface of the die becomes scarred, as if heavily polished. Some design elements appear blurry or doubled. Coins struck from worn dies rarely have collector value as die errors. Coins from worn dies represent die states.

Planchet errors

Alloy errors: All U.S. coins are produced today from alloyed metals that are mixed when molten to strict specifications. If mixed incorrectly, the metals may cool in nonhomogeneous form, with streaks of different metals appearing on the surface of the coin. Premiums vary, but tend not to be high.

Brass-plated cent: A post-1982 error is the brass-plated cent. Zinc planchets are plated with copper to form a copper-plated zinc planchet. Zinc planchets sometimes remain within the plating tanks and dissolve, contaminating the plating solution (electrolyte), adding their zinc content to the copper, thus forming brass. Subsequent planchets are plated with brass instead of pure copper. Brass-plated cents have a different color than copper-plated cents, although both can tone and may be difficult to distinguish.

Damaged planchets: Planchets are subject to various sorts of damage, including cracks (not to be confused with die cracks), holes and major breaks. Premiums for these kinds of errors are based on the severity and "coolness" of the planchet damage.

Fragments, scrap struck: Small pieces of coinage metal—fragments and scrap left over from the blanking process—sometimes fall between the dies and are struck. Fragments must be struck on both sides and weigh less than 25 percent of a normal coin's weight to qualify as struck fragments. Planchet scrap is generally larger than a fragment, and usually has straight or curved edges because of the blanking process. All carry nice premiums.

Improperly annealed planchets: This form of error, formerly called "copper wash" or "sintered plating," is found on copper-nickel 5-cent coins and copper-nickel clad coins. The coins are found with a thin, varying layer of copper on their surfaces, sometimes cracking and peeling, and they are often varied in color, including bright copper, red, brown, gray and black. New research indicates the copper is intrinsic to the planchet and is not deposited on it from outside sources as was once believed when the phenomenon was called by the older, discarded terms "copper wash" and "sintered plating."

Researcher Mike Diamond queried Mint officials about this form of error in late 2009 and received this reply from the Mint in early 2010:

"I've consulted with our technical experts regarding the cause of the appearance of the coins in the photographs attached with your e-mail.

"The samples shown in the photos were the result of a loss of protective atmosphere or being stuck in the annealing furnace for a prolonged period of time, or both.

"This would result in migration of the copper and nickel to the surface of the blank. Since there is three times as much copper [as] nickel in the outer layer of these coins, the diffusion of copper to the surface will be significantly greater than the diffusion of the nickel, resulting in the reddish appearance noted.

"Depending on the time the blank sits in the annealer, and whether it is exposed to oxidizing conditions, various reactions can occur. This will result in the type of phenomenon shown in the photos, where a distinct layer of material forms on the blank surface (primarily copper, with a high degree of oxidation), which is quite brittle, and will break off in pieces. This will expose the original blank surface, which would also be oxidized, but closer in color to the original alloy. We have seen these types of blanks but only infrequently."

Incomplete planchets: Often, though erroneously, called a "clip," an incomplete planchet results from a mishap in the blanking process. If the planchet strip does not advance far enough after a bank of punches rams through the metal, producing planchets, the punches come down and overlap the holes where the planchets were already punched out. Where the overlapping takes place, a curved area appears to be "missing" from the planchet. The word "clip," commonly used, suggests a piece of an already formed planchet was cut off, which is not the cause of the incomplete planchet. "Clip," when properly used, refers to the ancient process of cutting small pieces of metal from the edges of precious metal coins for the bullion; that is why U.S. gold and silver coins have lettered or reeded edges, to make it more difficult to clip a coin. Other incomplete planchets occur when the strip shifts to the side, causing the punches to overlap the strip's edge, or when the punches overlap the end of the strip. The missing area is represented by a straight edge, not curved. An "incomplete clip" occurs when the punch does not completely punch out a planchet, but leaves a circular groove. If the strip advances improperly, planchets overlapping the incomplete punch will bear a curved groove; the groove remains visible after the coin is struck. Most but not all incomplete planchet errors have a "signature" known as the Blakesley effect. The area of the rim 180 degrees opposite the "clip" is weak or nonexistent since the "clip" impairs the rim-making process in the upset mill. The lack of pressure in the upset mill at the missing spot results in improper formation of the rim on the opposite side. All carry premiums; value depends on the denomination and severity of the error.

Laminations: During the preparation of the planchet strip, foreign materials—grease, dirt, oil, slag or gas—may become trapped just below the surface of the metal. Coins struck from this strip later may begin to flake and peel since adhesion is poor in the location of the trapped material. The Jefferson, Wartime Alloy 5-cent coins are particularly susceptible to laminations, due to the poor mixing qualities of the metals used during the war metal emergency. Premiums depend on the severity of the lamination and the coin affected; many are inexpensive.

Split planchets: Planchets can split due to deep internal laminations or, in the case of clad coinage, because of poor adhesion of the copper-nickel outer layers to the copper core (these are also known as missing clad layer errors). Planchets may split before or after striking. On nonclad coins, the inner portion of the split shows parallel striations typical of the interior structure of coinage metal. The best finds are of both halves of a split planchet. All carry premiums and are nice errors. Thick and thin planchets: Planchets of the wrong thickness are produced from strip not properly rolled. Too little pressure can result in planchet stock that is too thick; too much pressure can result in thin planchets. Rollers out of alignment on one side create potential for tapered planchets—thicker in one area than another. Values differ.

Unplated planchets: New in U.S. coinage, unplated planchets became possible in 1982 with the introduction of the copper-plated zinc cent (and similarly in 1943 with the zinc-coated steel cents). The zinc-copper alloy planchets are plated after they are punched from the strip, but some planchets miss the plating process. Coins struck on the unplated planchets are grayish-white in color. Beware of Lincoln cents that have had their plating removed or have been replated after leaving the Mint. Major authentication services should be able to distinguish between a genuine unplated planchet and an altered version.

Wrong metal, planchet, stock: A wrong metal error is struck on a planchet intended for a denomination of a different composition. This includes 5-cent coins struck on cent planchets, cents on dime planchets, and higher denominations struck on cent and 5-cent planchets. A second type is the wrong planchet error, defined as a coin struck on a planchet of the correct composition, but the wrong denomination. These include quarter dollars struck on dime planchets, half dollars struck on quarter dollar and dime planchets, and dollars struck on other clad planchets. Some specialists claim that wrong metal and wrong planchet errors are striking errors, not planchet errors. Their argument? The planchet is OK. It was just fed into the wrong coining press. A third type is the wrong planchet stock error. It occurs when clad coinage strip rolled to the thickness of one denomination is fed into the blanking press of another denomination; the diameter is correct, but the thickness is greater or less than normal. The most common is a 1970-D Washington quarter dollar struck on planchet stock intended for dimes. A fourth, rarer, form is the double denomination. It occurs when a coin is struck on a previously struck coin, such as a cent struck over a dime. Since the U.S. Mint has struck coins for foreign governments in the past, it has been possible to find in circulation U.S. coins struck on planchets intended for foreign coins, as well as U.S. coins struck on previously struck foreign coins.

Another rare type of wrong metal error is called the transitional error. It occurs as a coin's composition changes. Some 1965 coins are known struck on silver planchets of 1964 composition, while some 1964 coins were struck on clad planchets. Until 2000, it was thought impossible for a coin to be struck on a planchet larger than normal (the larger planchet, it was thought, would jam the feed mechanism of the press). However, several exceptions to this rule surfaced in 2000, including State quarter dollars struck on the slightly larger Sacagawea dollar planchets. All carry significant premiums, with among the most valuable being double-denomination, transitional errors and, at the top, such coins as the 1943 Lincoln cents struck on copper alloy planchets instead of zinc-coated steel planchets.

Striking errors

Broadstrikes: If the surrounding collar is pushed below the surface of the lower die during the moment of striking, the metal of the coin being struck is free to expand beyond the confines of the dies. The design of the coin is normal at center, but as it nears the periphery, becomes distorted due to the uncontrolled spread of metal. All broadstrike errors carry premiums.

Brockage and capped die strikes: If a newly struck coin sticks to the surface of one of the dies, it acts as a die itself—called a die cap—and produces images on succeeding coins. The image produced by any die is the direct opposite on a coin, and brockages are no different. Since the image is raised on the coin adhering to the die, the image on the brockage is incused and reversed—a true mirror image. The first brockage strikes, perfect mirror images and undistorted, are most prized and carry the highest premiums. As additional coins are struck from the capped die, the die cap begins to spread and is made thinner under the pressures of striking, distorting its image. Although the image is recognizable, the design expands, producing an image that can be several times the normal size. At some point, as the die cap becomes thinner and the brockage image disappears, the coins struck cease to be brockages and are known as capped die strikes. Capped die strikes are simply a form of struck-through coin. At first, few of the original design elements from the obscured die are visible, though as the cap becomes thinner, more and more of the design elements are transferred from the obscured die through the die cap. Finally, the die cap breaks off or is pounded so thin it ceases to affect succeeding strikes. Sometimes, the die caps fall off early and in a relatively undistorted state. Die caps resemble bottle caps, with the metal wrapping around the shaft of the die. Die caps are very rare and collectible, much more so than capped die strikes. All carry premiums.

Double and multiple strikes: Double strikes are coins struck more than once. If the coin rotates slightly between strikes, but remains centered within the coining chamber, two images will appear on both sides of the coin. The first strike will be almost totally obliterated by the second strike, and the first strike will be flattened and have almost no relief. Sometimes, a struck coin will flip and fall upside down onto the surface of the die; thus, the second strike has an obverse image obliterating the original reverse, and a reverse image flattening the first obverse image. If the coin falls partially outside the dies after the first strike, the second image is only partial. The partial second strike obliterates the original image beneath it, but the rest of the first strike is undistorted, except in the immediate vicinity of the second strike. A saddle strike is generally not a true double strike, but usually the result of having a planchet fall partially between two pairs of dies on a multi-die press. Saddle strikes have two partial images and an expanse of unstruck planchet between the struck areas. Examples of coins struck three, four or more times are known, but are typically more rare than simple double-struck coins. All multiple-struck errors carry premiums.

Filled dies: The Mint factory has its share of dirt, grease and other lubricants, and metal filings. The recessed areas of the dies sometimes fill with a combination of this foreign material, preventing the metal of the coins from flowing into the incused areas. This results in weak designs or missing design details, and is among the most common types of errors. Filled-die coins are a form of struck-through error. Filled-die coins rarely carry significant premiums.

Indented errors: An indented error is a coin struck with another coin or planchet lying partially on its surface. The area covered by the planchet does not hit the die and thus is blank if indented by a planchet, or shows a partial brockage if indented by a struck coin. The most desirable of the indented errors are larger coins with the indentation of a smaller planchet centered on one side. Indented error coins are a form of struck-through error. All carry a premium.

Machine, mechanical or strike doubling: Machine doubling is often mistaken by nonspecialists as die doubling although the two forms of errors are unrelated. Machine doubling occurs at the end of the strike and is usually attributed to a loose die or a vibrating die assembly, although other proposed explanations may have validity, notes researcher Diamond. Specialists recognize two forms of machine doubling—"push doubling" and "slide doubling."

Diamond describes the appearances and causes of the two forms: "Push doubling leaves flat, marginal shelving along the edge of the design and rounded doubling on interior design elements. Push doubling develops when the die (hammer or anvil) bounces up after impact, shifts slightly to one side and lands lightly on the newly-struck design.

"Slide doubling smears the design. It occurs when a die shifts laterally without a bounce and, as a consequence, drags itself through the newly-struck details."

While many specialists find machine doubling uninteresting, some very interesting and collectible pieces have been found. Collectors should be wary of paying a premium for routine examples of machine-doubled coins, although the more exotic pieces may carry collector premiums.

Off-center coins: If a planchet lies partially outside of the dies during the striking, it receives an off-center strike. Each coin struck off center is unique. Off-center coins with dates are more valuable than coins without dates. Generally but not always, on dated coins, the greater the off-center strike, the more it is worth. Some collectors collect off-center coins by their "clock" positions. Hold the coin with portrait upright and look for the direction the strike lies. If it is at 90 degrees, the strike is at 3 o’clock; if it lies at 270 degrees, the strike is at 9 o’clock. Premiums have been rising for late-date examples of this error, since fewer are being produced and released.

Partial collar: Incorrectly called "railroad rim" errors; the edge, not the rim, is the portion of the coin affected. It occurs when the collar, around the anvil die is not fully extended, so that the upper portion of the coin is free to expand beyond the confines of the collar, while the lower portion is restrained. On coins struck from a reeded collar, partial reeding exists on the area restrained by the collar. The error gets the nickname "railroad rim" from its appearance—the coin, viewed edge-on, resembles the wheel from a railroad car. All carry a premium.

Struck-through errors: Struck-through errors occur when foreign objects fall between die and planchet during striking. Pieces of cloth, metal fragments, wire, slivers of reeding, grease, oil, dirt, wire bristles (from wire brushes used to clean dies, resembling staples), die covers, a thin die cap, other coins, planchets and other objects may fall between the dies and the coin. The most collectible struck-through errors are those with the foreign object still embedded in the surface of the coin. Value can be minimal to hundreds of dollars depending on the object or substance a coin was struck through.

Weak strikes: Weak strikes often resemble coins struck from grease-filled dies, but can be identified. They may occur when the press has been turned off—it tends to cycle through several strikings, each with less pressure than the previous. The error can also occur when the press is being set up by the operators, who test the placement of the dies at lower coining pressures, or if the spacing between the faces of the dies is greater than normal. On reeded coins, weak strikes generally have poorly formed reeding (it is strong on filled dies). Depending on the pressure used, the image may be only slightly weak, practically nonexistent or any stage in between. These coins are worth premiums.

Edge errors

Not long after the first 2007 George Washington Presidential dollars were released into circulation in February 2007, collectors, dealers and the general public began reporting numerous edge errors. The date, Mint mark and mottoes IN GOD WE TRUST and E PLURIBUS UNUM should appear on the edges of the Presidential dollars. The edge inscriptions on circulation-quality coins are added in a step separate from and following the striking of the obverse and reverse sides.

The most prevalent form of edge error on the Washington dollar coins is the missing edge lettering. Coins from both the Philadelphia Mint and Denver Mint were struck and shipped to the counting and bagging stations without being fed into the edge-lettering equipment.

Reports surfaced of 2010-D Sacagawea, Native American dollars with plain edges being found in 2010 Uncirculated Mint sets. The number of the errors released in the sets has not been verified.

Numerous other edge errors were discovered as well. One type involves multiplied edge inscriptions, with either partially or completely duplicated edge inscriptions found, some coins having the duplicate inscriptions in the opposite up and down orientation to each other. Another edge error involved improper spacing between individual elements of the inscription. Faint edge inscriptions were encountered, and on some coins, dropped letters from either the obverse or reverse were found impressed into the edge.

An apparently unique error – the design edge mule – was reported in January 2010. A collector found a Zachary Taylor Presidential dollar of 2009 with a 2010-D edge inscription in a roll of 2010-D Native American dollars acquired directly from the U.S. Mint. The Taylor dollar was the last of the 2009 Presidential dollars. This error probably occurred when a Taylor dollar became hung up somewhere between the coining presses and the edge inscription station. Later, when production of 2010-D Native American dollars was begun, the plain edge Taylor dollar was dislodged from its hiding place and became mixed in with Native American dollars being transported to the edge inscription equipment. The Taylor dollar then received the wrong year's edge inscription.

Another category of interesting errors involving the edge occurred when a number of unstruck planchets were edge lettered but not struck between obverse and reverse dies. These pieces are blank on their faces but bear lettered edges; they were found in rolls of Washington dollars.

Some Proof 2007-S Thomas Jefferson Presidential dollars were found with elements of the edge inscriptions out of sequence. Proof coins are struck with three-piece segmented collars that form the edge inscriptions at the time of striking. Mint workers created the Jefferson dollars with out-of-sequence inscriptions by installing the individual collar segments in the wrong order. On the error coins, the motto IN GOD WE TRUST on the edge is followed by the motto E PLURIBUS UNUM.