I had a good one the other day. Boarded a QF Dash 8 and was the last to board. Lady was in my window seat next to one largish guy on the aisle.
"What seat does your boarding pass say?" Turned out it was in one in front - that had precisely no one at all in either A or C.
"oh, don't get up, stay right where you are I don't want to trouble you. Completely happy to sit in your seat *mumbling-to-myself*
..and enjoy your shadow*
I've been there. Canberra to Sydney, most likely. I got in the wrong seat, settled myself down, and along comes a lady who glares at me and says I'm in her seat.
She was right, I was wrong, but my assigned seat was the one ahead with a shadow, and I couldn't see the point in stirring myself for one row on a short flight. On a Dash-8! I indicated that my assigned seat was a better one, and she was welcome to it. She thought this was a fair deal.
Those things, I usually just look out the window or read the whole trip, there's no need to leave my seat, and half the time I wave away the sugary, fatty snack and the lukewarm beverage.
Another time I was flying QF HNL-SYD, J in the nose cabin. I'd wanted a window seat, but couldn't find one, and at checkin the lady looked at the screen and said, "oh your seat is a window one."
I got aboard, turned left, found the row, got into the window seat. Those nose cabins have skew-wiff seating anyway. My first (and only) time in one, so it wasn't familiar territory. After a few minutes along comes another chap, looks at me, looks at the seat, climbs in beside me in the aisle and we sat like that the whole flight. I had a grand time looking out my window, enjoying the tropical vistas.
Halfway home I got up to use the facilities, got a good look at the seat numbers and realised that my assigned seat was in fact the aisle - not the "isle view" - and I'd been sitting in the wrong seat, which my seatmate must have selected well in advance for the window view I'd been enjoying.
The embarrassment still burns.
I wish the other guy had said something at boarding.
Sometimes it's not rudeness or "trying it on", it's bad information and ignorance.