A mule coin is a coin struck with two dies that were not intended to
"A numismatic mule is a fascinating creature, a hybrid of two designs never meant to appear together,” Coin World senior news editor Bill Gibbs wrote in 1992. "Some are accidental in nature, true errors that any collector would welcome in his or her collection. Other mules were deliberately produced, either accidentally, out of necessity to meet coinage demands, or from a desire to produce something special (even if fraudulent) for collectors."
The hybrid coins take their name, of course, from the hard-working animal that results from the cross-breeding of a horse and a donkey—half one thing and half the other.
There is a market for mules just like there is for correctly struck coins.
"Among U.S. coins, some mules have achieved legendary
status,” Mike Diamond wrote in a 2013 Collectors’ Clearinghouse column.
"The most prominent of these would be the 14 Sacagawea dollar
planchets struck by a Washington quarter dollar obverse die and a
Sacagawea dollar reverse die.”
Paul Gilkes wrote in 2012 that one NGC MS-65 example of the Washington quarter dollar/Sacagawea mule sold for $155,250 at a Stack’s Bowers auction.
Diamond’s 2013 column also touched on foreign mules.
"Probably the best known example among foreign coins is the 2000 Canada Millennium series 'Map Mule' 25-cent coin,” he wrote. "The obverse is the one normally used for this series and features the bust of Queen Elizabeth. The reverse face of each coin carries a design normally seen on the Royal Canadian Mint medal that accompanies the 12-coin 25-cent piece Mint set produced in that year. The design features a map of Canada constructed from maple leaves."