When on a mule, opening the inventory screen and trying to quick move items from held inventory to the mule's chest when the mule's chest is full (15 items) results in the elimination of the held item (or stack). I've lost a diamond axe and two maps (and several stacks of dirt and sand testing) this way. It effectively makes the mule's chest screen a garbage disposal if you're not careful. Elimination is instant and permanent. This was duplicated on a Realms world.