In numismatics, an error refers to a coin or medal that has a minting mistake, similar to errors found in philately. Because the U.S. Bureau of the Mint keeps a careful eye on all potential errors, errors on U.S. coins are very few and usually very scarce. Examples of numismatic errors: extra metal attached to a coin, a clipped coin caused by the coin stamp machine stamping a second coin too early, double stamping of a coin. A coin that has been overdated, e.g.: 1942/41, is considered an error.
Read more about this topic: Error