Was on an Emirates flight once. Someone turned up and said I was in their seat. Showed me their boarding pass and it was Indeed their seat. Got my boarding pass out and it said the same seat. The stewards were very confused. They moved me to business class as I was on my own and the gentleman had his partner with him so they say together.

A little later I looked at my boarding pass again and it was the boarding pass for my previous flight.. no one had noticed and only looked at the row and seat number not the flight number / destination.

I laughed but did admit it to the steward who also laughed at my stupidity and said she also checked on their system and had already clocked it. But they left me in business class.