I've accidentally gotten a little too trigger-happy (or should I say axe-happy) when various associates of mine have surprised me by showing up unexpectedly. I recently killed my follower, Gorbash Iron-Hand, that way in Blackreach. As its name implies, the place is quite dimly lit except for the eerie glow. I had lost Gorbash somewhere and thought he would stay lost. Then the poor bastard pops up unexpectedly out of the shadows to my left, and BAM! Whoops, I didn't realize it was you, Gorbash.. sorry bout the axe to the face.
Another time I really messed up when completing a quest for the Jarl of Markarth where I was supposed to kill a forsworn leader in one of the caves. I entered the cave, not remembering it was the very same cave where my associates from the Cidhna mine break had set up shop after they escaped from Markarth. So I enter the cave, and there is my old buddy Madanach and his cronies, all dressed up in Forsworn gear, of course. Before Madanach had finished his sentence, "ah, there's my friend from Cidhna.." BAM! Axe to the face. Sorry Madanach, I mistook you for one of those hostile Forsworn. No hard feelings eh?
For some reason they didn't quite understand why I did what I did and got a little hostile with me.
Didn't want to kill them all so I had to reload, enter the cave in peace, sneak up to the hostile Forsworn Briarheart at the back of the cave and kill him stealthily so I wouldn't invoke the wrath of my old chums.