I encountered this once in domestic F finding a woman in my seat, when I took out my boarding pass to double check, she said to me, "There was a mix-up with our seats. (unsaid: I have no intention of getting up)"; her husband was seated in the window seat next to her. I let it go and took her seat, rather than deal with sitting next to her hostile husband for a couple of hours. Next time I would call in the FA.