Did a quick search if it was a known problem but couldn't find anything. This happened to me just a few seconds ago: I was going up some stairs and I walked down one space. After that I don't really know what happened, but bottom line is: there was an enemy (a blobby) on top of the stairs (with me being one tile under him). I killed the blobby, and the game froze. I'm pretty sure it has to do with a corpse (or loot) being on top of the stair tile. Is this known yet?
