Inexplicable Instant Deaths.
Losing 200+ health in one hit from a dark orc, or from a jackal, etc.
Periodically as I;ve played this game over the past decade, something will happen where a robust character (100+ hit points) will be taken down into deep negatives (-200 hp) from one hit by a creature that shouldn't be able to do it. It was a Dark Orc today. But I've seen jackals and other creatures do it too. Last time was on R49. When I brought this up in years past, I was told that sometimes stuff like this just happens- a creature picks up a weapon of devestation, or happens to be in a room that makes this happen, or is spawned at a crazy high level for no reason.
I can't report the details of the bug, becase I'm just playing the game, and then I'm dead in one hit, and then my save file is gone,and then maybe it won't happen for a month or a year.
I think now that the game is going to Steam, it's time to tighten up situations like this so they are less likely. A permadeath game that's going for wide release needs to avoid this sort of situation.