I didn't see this listed in the summary, so I apologize if it has been listed here before:
Mercs get stuck and can't move or fight, although they are still shown on the screen and on the map. They are not stuck behind somthing or in a room, they are just completely stuck for no reason. The only way (other than save & quit) to set them free is to have your character enter a different location either by teleporting, waypoint jumping, or just walking into town or a different location in the game (where the game tells you it's a different place in the upper corner).
I noticed this bug in 1.09 a lot, especially with the Rogue merc, but don't remember dealing with it in 1.10 (although it may have been there). It's happened to me in Mac version 1.11b with three different characters already, all using the Rogue merc in Act 1.
Anyone else notice this, especially with other merc in different acts and/or on Windows?
Jude