I don't see this sort of thing that often, mostly passengers take the seat as stated on their boarding pass, but there was an incident on my flight to CPH yesterday.
Man, woman and baby, I think he was a GCH, he seemed to have reserved 7C and 8D. The logic being that this was at the front row of ET, 7C got him an empty 7B due to Theoretical Seating. So he put the wife/partner into 7C with the baby to take advantage of the empty seat, and he took 8D. Very sensible, so far, apart from a truly vast amount of cabin luggage worthy of its own thread, I think they had 6 or 7 bags in all. I guess they wanted to get off their aircraft quickly in CPH. This was on an A321 and regulars will now that row 8 is a 2+2 seat ahead of the exit row seat, is rock hard (no recline) and is subjectively fractionally more cramped than general seating. I would speculate he had at some point suffered the impact of Curtain Roulette and had originally wanted something like 7C and 7D. Or 7A and 7C.
Now this individual was a flapper par excellence, he must have gone into the overheads a dozen times during a flight that was only about 90 minutes long. He also brought out an extensive array of entertainment devices and headphones which needed constant juggling. I imagine the passenger in 8E - which is not really a middle seat - would have been getting quite irritated by him jumping around so much. After CE breakfast was finished, and probably in the final 20 minutes of the flight, the man decides to take over 6C, the last row of CE - which at that point was an block of CE seating. At the time I was talking to the SCCM, but she saw what was happening and immediately said "excuse me, I have to attend to this...." and went down to 6C brandishing her iPad. He did at that point ask if it was OK so that he could use the empty space and though he laboured the point somewhat, but the SCCM insisted and with bad grace he moved back to ET. I did feel sorry for the guy in 8E.
That's it, no swear words, no raised voices. I'd say this sort of thing happens one flight in 50 (?) or less.