This is quite a funny and epic happening I had earlier in the game. I had a fight with Master Vampire on narrow stairs and battle was going well: I needed only one hit to finish him and I still had over half of my health left. Well my companion decided to come to help me in the melee battle but there was not enough room for him to get in the next of the vampire. So he made awesome finisher to jab his glass sword through my back and then started to fight with the vampire over my dead body. Thank you Vorstag, I'm really happy I payed 500 gold for you to secure my back.

WTF!?! A companion stabbing you in the back!?!?! That's nuts!!!

Heh the funny thing I had was entered the last stage of vampirism and my chars wife turn on my orc.

Cosmach is better

It's not that unusual. I had Lydia turn on me in the middle of a dungeon once. I tried running away so that she'd stop attacking me and neither of us would have to die, but then she suddenly screamed "I cannot best you!", bolted in the opposite direction, spontaneously combusted and keeled over. At which point I got sick of putting up with her antics and decided not to reload from an earlier save. Just looted her body and went on my merry way.