When your having sex with a girl, your friend comes out of the closet and you swap so you friend is banging her. You quietly sneak out of the room. Then call another friend to come knock on the window. At this point she will be all confused how she is not fucking the same man anymore. Now you bust in through the door proclaiming, "What the fuck is this!?!?"

Literally everyone, ever. Including the writer of this definition.