A mule is a coin that results when two mismatched dies are used in the minting process. The types of mule coins are: intentional, emergency, error and double-denomination.
Only three error mules have been recorded in the American study of currency: a Philippines coin made at the San Francisco Mint in 1918, a coin that combines one side of a 1999 1-cent coin with the back of a Roosevelt Dime, and one that combines half of a 50-States Washington Quarter Dollar with a Sacajawea Dollar. "Mule" can also be used as a verb. To mule is to combine dies that were not meant to be used together.