ToME: the Tales of Maj'Eyal

This has happened to me twice. Both times, I was playing a Temportal Warden using a bow and had Weapon Folding up. On killing a monster, (a white crystal in Arena mode the first time, something I can't remember in Norgos' lair the second) it didn't die, but stayed present, not acting, its health going into negative if I kept attacking it (and becoming a long green bar extending from the monster in the negative direction.) The arena-mode one didn't disappear when the wave ended.

The second time it happened, in Norgos' lair, something even weirder occurred -- I went around the other way and eventually encountered Norgos. Then, retreating a bit, I came back to where the 'undying' monster was -- and it had turned into an exact copy of Norgos, displaying his health and everything. Attacking the real Norgos caused the displayed health on the Norgos-copy to drop, too. I attacked Norgos a few times; eventually, my game crashed to desktop with no error messages. I'm unsure whether the attack that caused the crash would have killed him, but I don't think so.

I forgot to mention, if it helps: Every time I hit the monster (once they were in negative health) it would display a death message for them, but, of course, they wouldn't die.