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.

One woman walked past me to (I guess) her ticketed seat, then came back and plunked down in J. A moment later, a passenger boarded who actually had that J seat, so the FA came to resolve it, and told the woman to go back to her seat in Y. She went as far as the second row of Y, and then as soon as the FA turned her back, went forward and seated herself in J again. Unfortunately, she'd picked one of the other 4 seats that had been assigned, so when that passenger arrived, she was again sent back by the FA. I think she tried a third time as well, but the FA recognized her by then and there was, umm, Words.

This attempt was particularly ludicrous on a flight of less than an hour during the day 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.