A while back on a LAS-EWR flight which had checked in full in F there was some sort of mechanical problem and all had to deplane for about 30 min. When we reboarded, one of the F passengers was missing. After many airport wide announcements and warnings--the FA brought someone on the waitlist up for that seat. Well low & behold, just as they were closing the door, Mr. orginal F seat shows up. The FA's refused to give him his original seat, he made a big fuss--but no dice. A nice comfy middle seat in the back was where he ended up.