Can monsters just vanish?

[Ebon]

I remember that I was playing a single player game. I was a sorcerer somewhere in dungeon level 2 or 3. There were lots of skeleton archer types grouped together in a large hall, all shooting at me. Very many. I had to take cover, and the curious thing is that when I returned to kill them, they were gone! All gone.

Can monsters really disappear? Is it an overflow?

[Serenity_]

I remember this happening in multi due to lag... sometimes if you go up and down the stairs they "reappear"

Theres also the barrel bug, where skeletons that come from barrels reappear when you go out and back in