My suggestion for a player event:

"Bored M.U.L.E. haxx0rz perform unauthorized surgery on your M.U.L.E.  They left behind 1 smithore in unused parts.".  One mule is randomly changed to a different type of mule, but the mule's owner gains some smithore that was left lying around.

This is annoying in that you now have to waste time changing the mule back to what it was (unless you like the change), but the smithore you find should pay for the alterations (depending on the price of smithore).