Amulet stealing

Quest nemeses and the Wizard of Yendor have a so-called 'amulet-stealing attack. Despite the name, the attack targets not only the Amulet of Yendor but the quest artifact, the Bell of Opening, the Book of the Dead, and the Candelabrum of Invocation (in that order of priority). On one-twentieth of the hits from such an attack, the monster will look for one of the above items in your inventory; if it finds one it will steal it and, if capable, teleport away.

The Wizard of Yendor is most notorious for this, as if he double troubles his double has a chance of carrying a cheap plastic imitation of the Amulet of Yendor which can be confused with the real amulet if it is stolen.