In numismatics, a mule is a coin minted with obverse and reverse designs in error and can make them highly collectible at often high premiums depending on rarity.
A recent example in early 2014 by the Royal Mint was the mix-up of dies resulting in a run of fine 1 oz silver coins where Britannia and Lunar Horse were incorrectly stamped and released before the errors were spotted. The Lunar Horses were struck with the denticled Britannia obverse while the Britannias were struck with the non-denticled Lunar Horse obverse.
Early sales on eBay saw prices of almost £1,000 for each of these coins but prices quickly dropped to the £100 or less.
There are still mules listed for sale with asking prices of several hundred pounds.