<font face="Verdana, Arial, Helvetica, sans-serif" size="2">Originally posted by ender83:
i was in the middle of my college graduation trip, leaving Israel to come back to the states on BA. i board in LHR and get to my seat in Y to find someone sitting in it. we compare boarding passes and we both have that seat number on it. a very nice FA comes over and (i guess since i was the one standing and seatless) asks me if i would mind sitting in J for the flight. i responded that i had paid for a Y ticket and expected a Y seat. yeah, right. 
i thanked her several times and enjoyed the fully flat bed seat in BA J from LHR-JFK. i think the lady in 'my' seat in Y was wishing she had boarded a little bit later!</font>
Had a similar experience, but I was the one seated! Couple of years ago, took my wife with me to SIN on BR SFO/TPE/SIN with her on an award ticket. We were seated in my favorite seats in economy deluxe in TPE when someone came in with the same seat as I on his boarding pass. Aha, he said, looks like an upgrade for me. The purser asked if my wife is with me, went away with everyone's boarding passes, then returned and asked my wife and I to go upstairs.....most likely because I was at BR's 2nd elite level, Gold. Later the purser came by our bulkhead seats and gave us each a small gift....which I have not gotten again even on purchased J and F BR flights.
For about a year to a year and a half, I was consistently getting op upgrades on BR on at least one segment per trip to SIN....conditions just right with almost full flights with room for an upgrade. Sometimes got it while boarding, sometimes after being seated, but that was the only time with the other seat occupant standing there.