I've seen this a couple of times; if an enemy is hanging out at the very top left corner of a room, sometimes they can get stuck there, facing the wall and not moving or attacking. They usually free themselves after a while. Mages will also forget to heal things, which is nice :)
I'll try to get a screenie next time.
Monsters seem to get stuck on anything they can, and sometimes seems hesitant to walk across a (down) gate. Monsters will sometimes, even if they don't appear to be stuck, sit there with their back to me and let me shoot them. I'm not sure if I should be /bugging when I see this? It happens very often.
Also, this isn't a bug, but many of the maps seem to be set up in a way where I'm supposed to be trapped in an area with recently spawned monsters. I can easily manipulate it so that there is a half-wall between me and them, and I can just shoot them until their dead. Even enemies with projectile attacks, my gun shoots over the half-walls but their attacks don't. I don't know if this is a concern, but I seem to have a lot of success as a result of manipulating the enemies beyond what appears to be the intent of a level.