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?
I have seen one of my spirits get stuck in an enemy rooms -- they couldn't pass through their portcullis. I had assumed they followed an enemy in there and couldn't get back out. I don't have a screenshot to share because I didn't assume it was a bug. Sorry about that. I know they can pass through doors, but I didn't know if portcullis count as a door because they don't count as a fortified wall like doors do in determining room efficiency.