The most hilariously clumsy attempt I've seen was on a very lightly loaded (~15 in Y, and ~5 in J) A320 ZRH-GVA mid-morning. I was seated in the front row of Y and was one of the first to board, so I got to watch the entire drama.

The woman who boarded after me took a J window seat. Another passenger arrived and looked confused. FA looks at first woman's BP, points to mid-Y somewhere. She heads back, stands in third row of Y until FA's back is turned, goes forward and takes a seat near the back of J. Next J passenger looks at her, his BP, calls FA, FA sends the woman back to cattle class again. This was repeated at least twice more, as the woman kept trying to take J seats and getting found out. After everyone had boarded, she had one more try. The FA doing the final check before takeoff saw her and there were, umm, Words.

This attempt was particularly ludicrous on a daylight flight of less than an hour when each Y passenger had a full row of seats to themselves; that is, there were no significant benefits to being in J compared to Y.

I think the funniest thing was her bad luck in only picking the J seats that had been allocated.