On our LAS-SEA flight 11/16/04, there was a bit of an argument behind us, with two passengers claiming seat 19C. When the FA arrived to straighten it out, the person who was there originally was not supposed to be on our flight, and from the little I was able to hear, was not even travelling to SEA. How does this happen in this day and age? I thought the boarding pass scan would have shown the 2nd person checking in that the seat was already taken, raising the red flag before the argument commenced. And, that the scan meant something. Is this a common occurence? Other than that, nice flight.