ToME: the Tales of Maj'Eyal

Steps:
1. go downstair
2. cast inferno
3. go upstair and rest
4. go downstair and cast another inferno
5. repeat above

you will have multiple infernos incinerating the level at the same time

More like an exploit then a bug. Maybe some day creatures will follow players when they stair scum.

If monsters on one level healed when you rested on another level, it might not be too advantageous to stairscum... In any case, if there's a mob surrounding the entrance, what other strategy is there?

Well, the "bug" part of this is the fact that spells on another level don't time out when you spend time off their level. You can spend a thousand turns off the level, then come back, and the spell's still there exactly as it was when you left.

Time on dungeon levels is frozen when your character is not on them. It's almost the case in all roguelike I played (ADOM has the same feature).

I encountered a particularly nasty version of this: I went down stairs, casted tidal wave and ice storm, went up, then went back down, then got killed by my own tidal wave and ice storm.