First, there's a distinction between someone with a Y ticket taking a J/F seat (self-upgrade) versus someone taking the aisle rather than the middle in the same row, versus someone taking an equivalent seat (aisle seat on the opposite side or one row back).

I've only seen a self-upgrade on a 50 minute flight ZRH-GVA; the FA sent the passenger back to Y and the passenger came back to a different J seat. 4 times.

I've seen lots of isolated "right section/wrong seat" incidents outside the US; frequently it's an accident (off by one row) or hopeful (sit in the aisle rather than the middle until someone shows up). I once sat in 39C (aisle) rather than 39D (aisle) due to jetlag. Outside the US, I've always seen these resolved by either the person in the wrong seat admitting their mistake, or by swapping equivalent seats (the guy with the BP for 39C took my 39D seat rather than make me move.)

I will say, though, that it was in the US that I witnessed an impressive if frustrating multiple seat poach shuffle. It was the last three rows of Y (say, 23 to 25) on a 737 LAX-CLE. Mr 23B arrived first and took 23C instead. Ms 24E decided to snag 24F. Mr 23C, finding his seat occupied, was persuaded by 23B to take 24C instead. And so on - each successive passenger was told "I'm supposed to be in [ ] but he took my seat so I'm here now so why don't you go over there." RadioBloke and I were among the last to arrive (short connection) and eventually worked out that NO ONE in those rows was in the right seat and NO ONE was willing to move. (RB and I were just off a SYD-LAX, so we weren't too sympathetic to the locals complaining about the 'long flight' to CLE, but that's another story.)