This happened on Adom for Windows, version 1.2.0 p23.

PC threw rune-covered trident at a Chaos warlord and wasn't able to catch it again. So the trident ended up on the same tile as the monster. Monster picked up the trident and tried to equip it, but could not unequip its shield first in order to do this, I assume because the shield was cursed. I unfortunately didn't copy the actual message. After immediately killing the monster the trident was nowhere to be found.