I ran into a real doozy the other day at ORD. Apparently this guy had been FIM'ed over from an American flight that had cancelled, and had upgraded into an F seat on his AA flight using miles or status or whatever they do over at AA. In any event, it was clear he wasn't flying revenue F or J class and AA had authorized only a Y seat on AC.
This guy was going on and on about how he was supposed to be in "first class", started pestering the J pax about whether there was anyone sitting in the empty seat next to them, and holding up the boarding process all the while. He demanded to be put in one and wouldn't move until he was. His argument, in a nutshell, was that he should be put in the seat because it was empty.
I was embarrassed for the guy, as was everyone else... strangely, he was totally unrepentant about the spectacle.