Spirits can be trapped by AI between two doors
I noticed it in the level where you have to protect a weaken underlord from waves of dwarves. Enemy AI usually locks doors when your creatures approach them. In that level there is a small tunnel at the heroes territory between two metal doors (don't remember their ingame name :)). So somehow one of my spirits got there, AI locked both doors because the spirit was too close to them both, and the spirit just remained there, unable to go anywhere without my help.
I dunno if it's a bug or an intended feature, but aren't spirits supposed to be unstoppable?