A mule occurs when a coin is produced with mismatched dies. In some countries, the Mints use dies for different denominations with the same dimensions. In these instances, mules have been known to exist and are not that rare. Mules struck on patterns are extremely rare.
Pictured above is a proof Half Sovereign off-metal mule pattern that encompasses a combination of history, rarity and condition. This piece is the ONLY known certified example and the ONLY known piece as well. The obverse was struck from the regular obverse die intended for the Sixpence issue. The reverse was struck from an experimental reverse die, intended for the proof Half Sovereign issue.