I had the opposite around a month ago, flying DFW-LAX on the last flight of the evening. We started boarding really early and it was lovely and relaxed, with a great crew doing PDB (in F) and making lots of welcome and boarding announcements. Why can't it always be like that...? Anyway, the FA made a clear and concise announcement that anyone not going to LAX should get off, a few minutes before door close. The doors then closed and a few minutes later we started pushback. A minute later, a guy comes running down to the front with his suitcase, stating he was on the wrong flight. Uh-oh, I think, my connection in LAX may now be at risk. But no, we pulled back to the gate, the door opened, the guy got off, the door closed again, and we were pushing a minute later. Total delay was 5 minutes. Impressive. He wanted the PHX flight which was departing from the adjacent gate...