I had it happen to me twice just a few months ago. To be fair, I was on a tight connection and my originating flight was delayed so it wasn't clear to the GA that I'd even make the flight. When I did show up at the gate just as they were closing the gate door (they hadn't closed the aircraft door yet), the GA asked me to hold a moment while she checked on the situation. She verified the aircraft door was still open then led me on, saying my original seat wasn't available anymore but there was one further back in the aircraft. At that point, I was just happy to make the flight. Barely made the connecting flight from THAT flight and got a new seat assignment for it only to find it occupied but the guy who had slid out from his middle seat into my newly-assigned aisle seat slid back in.