In numismatics, a mule is a coin or medal minted with obverse and reverse designs not normally seen on the same piece. These can be intentional or produced by error. This type of error is highly sought after, and examples can fetch steep prices from collectors.
The earliest mules are found among ancient Greek and Roman coins. Opinion is divided between those who think that they are accidental, the result of an incorrect combination of a new die with one that had officially been withdrawn from use, or the work of coiners working with dies stolen from an official mint, perhaps at a time when one of them should have been destroyed.
The name derives from the mule, the hybrid offspring of a horse and a donkey, due to such a coin having two sides intended for different coins, much as a mule has parents of two different species.
Several prominent mule errors have been discovered in recent times. One of the most famous is the Sacagawea Dollar/Washington State Quarter mule featuring the obverse of a Statehood quarter and the reverse of a Sacagawea dollar. This coin was struck on a Sacagawea dollar planchet. Common belief was that this coin was intentionally struck by a mint employee, however the mint confirmed in July 2000 that the coin was a legitimate error, created by the accidental replacement of a cracked Sacagawea obverse die with a Washington obverse die. Several thousand of the coins were reported to have been minted before the error was discovered, and mint employees recovered and destroyed most of them. As of July 11, 2011, eleven are publicly known to exist and have been certified.[1] One of the few that escaped detection was sold on eBay for $41,295.
In February 2009, Coin World reported that some 2007 Abigail Adams medals, from the U.S. Mint, were struck using the reverse from the 2008 Louisa Adams medal, apparently by mistake.[2] These pieces were contained within the 2007 First Spouse medal set.[2] The U.S. Mint has not released an estimate of how many mules were made. Ebay prices in March 2009 were reported as high as $925.99.[3]
In 1967, a New Zealand 2 cent coin was issued, featuring the obverse of the Bahamian 5 cent coin, see Coins of the New Zealand dollar.
In June 2009 a rare dateless British 20 pence mule was reported to be in circulation, resulting from the accidental combination of old and new dies in production following a 2008 redesign of UK coinage, with an estimated 50,000 to 200,000 mules released before the error was noticed.[4][5]
The Winter Olympic coins produced in the Royal Canadian Mint Olympic coins program for the 2010 Winter Olympics in Vancouver featured several mules which entered circulation.[6]