So I'm happily clearing the library with my extremely imba human wizard who was about to win a neutral ultra. While clearing the hidden southern corridor with books the game suddenly is gone and I'm looking at my desktop. Not any message was generated or something :(
Can't recall I was fighting anything special in particular. But on the other hand I was pretty much afraid of nothing and there is lots of special monsters in the library so maybe it's monster-related or something...

Character lost. Lvl 40, wish to spare, all stats 40+ (except 1 appearance), immune to all elements, everything set for ultra. Already had all orbs + needle + medal of chaos. I was looking forward to attempting a GUV with 1 lich emperor on D46 as I had no trouble at all with Lich kings so far.