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)