Not really a DYKWIA, but more a case of not really knowing any different I suspect. Still, it provided me with a "wait for it" moment.
The early flight to NCE the other morning wasn't particularly full, and especially in Club, where there was just one person in the 5 rows. I was in row 6, and had a family taking up the rest of the row. The young lad was desperate he wanted a window seat so the dad told him to wait until after take off and go and sit in one of the spare seats in front of the curtain, as "they wont notice".
Well, sure enough as soon as the seatbelt sign went off, the young lad gathered his things and installed himself in row 5. This lasted the grand total of the amount of time it took the crew to walk from the front galley to close the curtain, where the young lad was put back into his original seat.
He came back and said to the dad that it was business class, so couldn't sit there. The dad muttered "ridiculous" under his breath.
To be honest, I don't see why the crew couldn't have let him stay put, and just not fed him, seeing as he wasn't causing any trouble - and it would have meant the middle seat in my row would have remained free. So much for theoretical seat blocking when there were plenty of spare seats in Y too... oh look, turns out I'M the DYKWIA in this story!!