> All the alternatives I can think of are extremely time-intensive - you'd have to go back to everyone who already scanned in and check their id against their b

> All the alternatives I can think of are extremely time-intensive - you'd have to go back to everyone who already scanned in and check their id against their boarding pass. That would add significant delays in taking off, which could cost the airline a lot of money in ripple effects throughout the day and will certainly cost the gate agents a lot of hassle and ill-will from customers. I've had to do exactly that before. Whole plane: deplane, re-scan boarding passes, re-board. Somehow we had one too many people on the plane on a full flight, exactly like this scenario. Of course it was the one guy that was making so much noise about "who gonna get in trouble" and "this [is] like a reality show."