Even today, with all the technology of the modern world, the minting process is not perfect. Frequently, a mint will commit errors in the minting process. These are referred to as “mint-made errors.”
Note that in numismatics, the term “variety” typically refers to types of coins with both intended and unintended characteristics. On the other hand, an error is almost always and unintended consequence of a minting process. There are three basic categories of mint-made errors: Strike errors, hub-and-die errors, and planchet preparation errors.
A coin is struck when a die, bearing the coin’s design, comes into contact with the planchet, or blank (a piece of bare metal). Sometimes, this process goes awry. There are many types of strike errors. A coin may be simply be struck off-center. Sometimes, two planchets will be fed into the machinery at once, resulting in two “uni-face” coins: one with only an obverse, and one with only a reverse. Brockage, one of the more valuable types of strike errors, occurs when a struck coin fails to eject, and another planchet is placed in between the struck coin and the die. While the new planchet is struck correctly on one side, its other side is impressed with a mirror image of the design from the struck coin!
The other two categories of errors result from problems with either component of the striking process: either the planchet or the die itself. Hub-and-die errors are generally the most valuable type of errors. An error with a die (or a hub, from which a die is created) often creates multiple error coins, which are noted by collectors and thus attain higher value. The famous 1955 Doubled Die Lincoln cent is an example of a die error. Planchet errors include cladding flaws (where one layer of metal has peeled or is otherwise defective) and clipped planchets (where a planchet itself is punched out off-center, resulting in a misshapen blank), among others. They are usually less valuable than hub-and-die errors.
Mint-made errors can be extremely valuable. As with other coins, the value of mint-made errors depends on the condition of the coin and the type and rarity of the error. Brockage, doubling, overdates, and coins struck on incorrect planchets are often the most valuable. Other types of errors are more easily faked, making them less valuable.
Historically, coins were struck by hand (i.e., without the aid of powered machinery). Likewise, dies and other pieces of minting equipment were very crude by the standards of today. Coins that deviated from their intended design were the rule, not the exception. As a result, errors on most ancient and medieval coins are very common and usually harm the value of these coins, rather than enhance it. The most valuable mint-made errors were produced in the era after the introduction of powered machinery, when perfection was more attainable and errors became rarer.
Theoretically, because of modern technology, mint-made errors are statistically less likely today. However, because modern circulation coins are struck in such large numbers, mint-made errors from the most recent era are also generally less valuable.