I had an interesting experience several years ago. I was flying long haul Y. I had booked myself an exit row seat.
Boarded reasonably late in the process only to find another pax in my seat. The obligatory "sorry, I think you're in my seat" comment. He pulled out his BP that shows him in that seat. My BP also shows me to be in that seat. We had similar but not identical names as well (think "Grem Lin" vs "Grem Lang").
FA wanders past and tells me to take my seat. I said that we both had a BP for the same seat. She commented that one of us must be looking at the BP for our connecting flight. We both show our BPs. She stares at one, then the other, then back at the first, then at the other and then says "I've never seen this before". Calls the cabin supervisor over who then goes to check the manifest.
Manifest shows me in the exit row and the other guy in a middle seat towards the back. THAT seat was also currently occupied by a standby pax who had received a late call up at the gate.
Lots of too-ing and fro-ing. Flight held up best part of forty minutes but ultimately I ended up with my exit row seat, "Grem Lang" ended up unhappily in a middle row at the back and standby pax, most unhappily of all, trudged off the plane and back up the aerobridge.