Only thing that comes to mind was once in L4D2 I was respawning as infected (to a ghost), became a spitter momentarily before one of the AI's on the other team teleported to my ghost (which was a pretty good distance away) and he started getting killed like I had charged him. He didn't take any damage, and his friends couldn't kill me since I was still a ghost spitter. I bunch of commons rushed him but eventually a melee knocked him out of my ghostly hands.
I don't have a video of this, but my friend and I did manage to nab a couple screenshots:
http://steamcommunity.com/id/Blade1130/screenshot/630733938201479791?tab=public
http://steamcommunity.com/id/NijiharaInk/screenshot/594705141182045918?filter=app_550
http://steamcommunity.com/id/NijiharaInk/screenshot/594705141182063067/?filter=app_550
I have no idea what triggered this and have never been able to replicate it, though another guy on the server mentioned that he had seen it happen before. I think he was lying though...