I once boarded a flight from Montego bay to Kingston with a friend. We had been travelling for around 40 hours and were unaware of a hour's time difference from previous airport and arrived at the gate an hour early. bags were checked and a security guy dropped an expensive camera just in front of us. I guess that's why they didn't really look at our boarding passes. On board I found my seat occupied and thought I was getting an upgrade because the flight was overbooked but they kicked us off because we were on a flight back to the US. My friend's seat was free and if mine would have been too I think it would have taken at least an hour before I would have realized I was on the wrong plane.... These days boarding passes are scanned...