A few years ago I boarded an Aer Lingus flight at Dublin for Gatwick. Chap sitting next to me was settling down, with seatbelt on, when another man arrived at the last minute. Both of them had a boarding pass showing the same seat number and some 'animated discussion' followed. After intervention by the cabin crew, it turned out that the seated man had a boarding pass for an entirely different Aer Lingus flight, that had left 30 mins beforehand. None of the boarding checks had picked this up, and, had the seat not been booked, the seated man would never have known his mistake until after takeoff. (Have no recollection where he had actually intended to go)
Many years ago (pre 9/11) a friend who worked at a business consulting firm (lots of short notice trips etc) woke up late and just made it to the gate in time to board, when he got to his seat there was someone in it with same # on boarding pass.
Flight attendant sorting it out ended up telling my friend "sir you have this seat tomorrow, this gentleman has it today". Instead of almost late he was a day early. I believe the boarding pass came from in-house travel agency, but not sure.
He also once (again pre 9/11) made a flight by getting out of his car in passenger drop off zone and running in to the terminal, policeman yelling at him "you will be towed" etc. It was worth the fine and storage fee to not miss a meeting, basically valet parking by state trooper.
Of course if you tried that today you would at best be tasered and tackled...