Probably when I and my Followrer, Lydia, was in a major fight in a dungeon against all these undead and then a final undead knight which tok ages to kill. I had to run all the way back through the dungeon trying to delay it on traps. Finally I archered it to death froma safe spot. Took ages.
Lydia was nowhere to be seen. eventually found her among all these skeletal bodies.
I took her and bought her to the end of the Dungeon where I left her propped up against a wall... And left her there in respect. I would`ve taken her out of the dungeon, but the game wouldn`t let me.
That was kinda sad. I know it`s just a basic npc of nothingness, but one can get attached to anything that`s been helping you fight for a long while.