The craziest floor feature I've ever found in game was an isolated room. As in it had no hallway leading into it! You had to use either Phase Door or Teleport just to get into the room. And I've only found it once; if I'd known it was so rare, I would have saved on that floor. I've seen several other crazy floor features.
Anybody else find any other crazy floor features; ones that are unusual and don't occur too often?