Mule

A Mule error coin is a coin that has been struck by two different dies. For example. A penny that has been struck correctly on the front but the back was struck with a dime's die, thus creating a coin made from the material that a penny is made from and the front design is the correct one but the back is not. This is but one example of a mule coin error and there are many others. The example mentioned is extremely rare. In fact at the time of this writing, there was only one known to exist and it is in a private collection. There have also been reports of the Sacagawea dollars with statehood quarters stamped onto the back.