I too have mistakenly sat in the wrong seat (and had the same happen to me) - all sorted quickly and amicably.
An odd one was 4 or 5 years ago PER-MEL on VA in one of their 'original' ex-EK 332s. I had 2D allocated (LH aisle of middle 3). When I reached the plane I realised a 737 had been subbed in. Now luckily 2D also exists in the J cabin of a 737 (a secondary reason for picking that seat, and there were definitely pax who did not make the transition from 332 to 737 because it was full). When I showed the FA my BP she turned and called to a chap who was already in 2D, something like "he's turned up, you'll have to move". Odd, but then he moved, or made to move, to 2C. I asked if that (2C) was his allocated seat and he said yes, so I said you're welcome to remain in 2D if you wish, and I'll have 2C (I prefer aisle) - he was cool with that. Of course, a minute or two later, the real 2C shows up! So I got the impostor to move out of 2D, I slipped into it, and the real 2C took his seat. I've no idea what happened to the impostor, whether he ended up elsewhere in J (I did not look back, I now wish I had've), or possibly back to Y where maybe he should have been all along. No idea how he came to be in 2D in the first place, perhaps deadheading staff, or a friend of the FA (based on her original comment to him when I showed up). Will never know.