This is the first really serious bug that I've noticed; fortunately, the likelyhood of it ever occurring again are minimal, since the circumstances were rather specific.
My party consisted at the time of a Hirebrand, a Priest, and a Thief. There were more, but they were not involved.
My Priest casts Hold Person on a monster group; he's on the front line so he needs meatshields. Problem is, after I selected the target group and the spell is cast, he is killed, and the targets of the Hold Person spell attach themselves to my Thief who steps into his spot! I guess someone had to take control of those poor souls, but it still strikes me as something of an error.