As with any product, there is margin for error. Errors are educational because they graphically show what can go wrong at a mint. But because error coins are the exception rather than the rule, they can be quite collectable. People who collect error coins are called error collectors. Error coins can be collected as error type sets, date sets, or even what one collector calls a “looks cool” set.
Here is a list of coinage errors you might encounter.
Broadstrike: A broadstrike is a coin struck without a collar around the coinage blank, resulting in a coin that is not perfectly round. This coin will typically lack good centering and detail.
Brockage: A brockage is similar to an indent (when two coinage blanks get into the coining machine at the same time, causing one to overlap the other), however with a brockage a struck coin is struck onto the blank, leaving a distorted but mirror image impression.
Caps or die caps: Coin blanks occasionally stick to the coinage die when they are struck. When the die slams down on the next blank ejected into the coinage machine, the metal flows up around the die and the resulting coin looks more like a bottle cap. These are typically caught by mint quality control and for that reason are desirable among error collectors.
Capped die strike: If the coin press continues to operate despite a coin now being stuck to one of the coinage dies, as further coin blanks enter the machine the additional blanks will either be produced blank or with only varying degrees of mush-like detail until the capped die coin finally either falls off or disintegrates.
Clashed dies: A coinage blank is supposed to enter the coining machine every time the obverse and reverse dies come together to produce the desired coin images on both sides of that blank. When a blank fails to enter the mechanism and the coining machine continues to operate, the two dies will clash, leaving a partial image of the opposite side on each of the coining dies. When coinage blanks now enter the machine, this clashed image will be transferred to the coins that are produced.
Die break: A die break is a flaw in the coinage die in which a piece of the coinage die is missing. The blob of raised metal appearing on the resulting coin is called a “cud” or “die break.”
Die crack: If a coinage die begins to wear and develops a crack, that crack will be transferred to coins struck from that die. This will appear on such coins as a thin, raised line. As the die continues to disintegrate, the die crack will extend further on coins the die strikes later during this degeneration.
Double denomination: When a finished coin of a particular denomination for some reason goes back into the coining machinery and is struck a second time by dies producing a different denomination, a double denomination error will occur. The image of both denominations must be partially visible.
Doubled die: This is one of the more celebrated errors, especially regarding the 1955 Lincoln cent. This error occurs when the hub from which the working coinage dies rotates or shifts so that the doubling is transferred on to the coinage die. Coins struck from this die will have the same doubling.
Indent: When two blanks get into the coining machinery at the same time, one overlaps the other. This causes a partial strike with an indented blank area on the bottom coin.
Lamination: If the coinage blank begins to peal or crack after it has been struck to become a coin, this is because of defects in the host blank. This pealing or cracking is called lamination.
Mechanical doubling: While this type of error is often confused with the doubled die error, it is not a rare doubling effect. Mechanical doubling is caused by a loose die that twists slightly at the moment of striking the coinage blank. A flat shelf-like doubling results. A coinage die that has been overused until it is significantly worn will also result in mechanical or machine doubling, likely with weakly defined detail.
Off-center: An off-center error is a coin struck without a collar, causing part of the detail to be off the coin. Off-center strikes are more desirable when the surviving image includes the date, since there are collectors of these who seek them by date.
Off-metal strike: This is one of the more heralded coin errors. If a coinage blank meant for a different coin somehow gets into the hopper from which coinage blanks are being fed into the coin striking machine an off-metal strike occurs. The most famous of these is likely the 1943 copper composition Lincoln cent, a coin that was supposed to be produced of zinc and steel that year during World War II.
Partial collar: A coin struck with only part of the collar holding the blank will result in an area of the edge of the coin expanding out of shape. On coins with reeded edges this will give the appearance of a two-level railroad wheel, known to collectors as a “railroad rim.”
Split planchet: A split planchet error is a progression experienced by a severe lamination problem. The defect in the blank causes the coin to split into two halves.
Struck through error: A foreign object such as a piece of cloth, string, or even machinery grease may get between the die and the coinage blank, leaving an impression or causing the finished coin to lack part of the intended impression.
Uniface: When the image of only one side is struck onto a coin, the coin is uniface. This error is typically caused by an obstruction blocking one of the dies from striking the coinage blank.
Weak strike: This is a coin lacking detail due to insufficient pressure having been applied to the coinage blank by the coining machine. Do not confuse wear with weak striking quality.
Coinage blanks, or planchets, are made by punching blanks from a metal sheet of the appropriate thickness. There are three types of clipped planchets—curved, straight, and ragged. If the sheet is not properly advanced, the punch may overlap a hole, producing a blank with a circular clipped edge. This is called a clipped planchet. Beware that this type of error can be artificially created outside of the mint. Genuine errors of this nature do not have a raised edge of metal bordering the missing metal, and the coin image detail bordering the area of missing metal will not be sharply defined.
The second type of clipped planchet is the straight clipped planchet. This happens if the missing part of the coinage blank is the result of the sheet shifting toward its edge when the blank was to be punched. The third type of clipped planchet is the ragged clipped planchet. This happens if the blank was cut from a ragged edge of the sheet from which the blanks were being punched.
As with any coins, the collector value of error coins is determined by supply, demand, and the eye appeal of the error. The more graphic the error, the more valuable it typically will be.