No save file, the game ate it when it crashed. Incidentally, the same bug with the walking animation happened earlier. Oh and it's Win64 incidentally. I forgot.
That's really weird, don't think we've ever seen this before. I'll refrain from making the obvious reference here cause it sucks that it happened. Well, without save files we don't really have anything to go on, unfortunately.
It indeed IS weird. Personally, my experience is that usually the game just crashes when you try to go down the stairs and doesn't load the next level at all.
This one baffled me for a while, but I think I finally figured it out. Did my save game get eaten by a grue?