Mint-made errors

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Mint-made errors occur when coins are made incorrectly at the mint, including anything that happens to the coin up until the completion of the minting process.^[1] Mint error coins can be the result of deterioration of the minting equipment, accidents or malfunctions during the minting process, or interventions by mint personnel.^[2] Coins are inspected during production and errors are typically caught. However, some are inadvertently released into circulation. Modern production methods eliminate many errors and automated counters are effective at removing error coins.^[3] Damage occurring later (post-mint damage) may sometime resemble true mint errors. Error coins may be of value to collectors depending on the rarity and condition. Some coin collectors specialize in error coins.^[4]

Errors can be the result of defective planchets, defective dies or the result of mistakes made during striking. The planchet, die, and striking (or PDS) classification system happens to correspond with the mintmarks of the three largest U.S. mints, Philadelphia, Denver, and San Francisco. Some errors have multiple causes and not all errors fall neatly within the categories. For example, design elements may be missing from coins because die crevices were filled with grease — a problem with the die but the error occurs when the coin is struck. Labels used to identify specific categories of errors may describe the cause of the error (die crack, rotated die, clipped planchet), the appearance of the coin (wavy steps, trails, missing element) or other factors (mule, cud, brockage). Some errors are known by multiple names, e.g. filled die errors are also known as missing design element errors and as strike throughs.

Some errors, such as an off-center strike, are unique. Other errors, such as those resulting from a specific die crack, form a variety, i.e., a group of coins with distinctive details or characteristics. Uniqueness does not necessarily make an error coin valuable. Although no other coin may be the same as a coin with a particular off-center strike, off-center strikes of varying degrees are not extremely rare. Accidental error coins are perhaps the most numerous, although in modern minting they are rare, making them potentially valuable to collectors. Intentional intervention by mint personnel does not typically involve a deliberate attempt to create an error, but usually involves an action intended to improve quality that miscarries.