Each year, the U.S. Mint produces billions of coins. The vast majority are struck correctly and enter circulation or end up in collectors’ hands. In order to leave the U.S. Mint facilities, the coins must pass through a series of quality control checkpoints. If a coin is found to be struck with an error in the design, or if the coining machinery malfunctions and produces misshapen or partially-struck coins, all of the coins in question are destroyed.
However, each year a very few rogue coins manage to escape the eyes of inspectors and are accidentally released into circulation. These error coins are highly sought after for their rarity as well as their unusual nature. Due to their rarity, error coins can be expensive.
One major type of error coin is an error in the design. The coin will look normal, but a small flaw or unintentional element on a single coin die creates a coin that is different from all others. During a mint run for a coin, hundreds or even thousands of different coin dies may be used, but the error will appear on only one die. As a result, this type of error will appear on only a few thousand coins (at most) struck with the same coin die. Famous examples include the three-legged buffalo on the 1937-D Buffalo Nickel, which was the result of an overzealous Mint employee polishing one coin die so much that he rubbed the buffalo’s leg away; the 1955 Lincoln Penny with the doubled-die obverse, the result of the lettering and date being accidentally imprinted twice; and the 2004-D Wisconsin State Quarter with an extra leaf in the design
Another type of error coin can be produced during the minting process on otherwise normal coin blanks and with normal coin dies; these errors are usually unique, meaning that only one coin at a time will be affected by a flaw in the minting process. For example, a coin can sometimes be struck multiple times and will therefore show parts of two or more images; a coin can be struck off-center; a coin can be struck on the wrong type of coin clank; and a coin can miss the coining machinery and be completely blank. In addition, some 2007 and later Presidential Dollars were released with missing inscriptions on the edge.