I posted a longer version of this in a TR a while back, but I was on a JFK to PHX flight with AA when a woman boarded and stated that the woman sat opposite on the opposite side of the aisle in row 4 was in her seat.

The "You're in my seat" No this is my seat" went back and forth for a bit, and then the crew got involved. The woman in the seat kept stating it was her seat and refused to show her boarding pass, right up until the crew member stated if she did not hand over her boarding pass she would be removed from the plane. Boarding pass was reluctantly handed over.

I don't remember the exact row now, but it was something like 26.

After being admonished by said crew member, she walked back down the plane without a word.

I don't understand the thinking behind that to be honest, what did she think the woman who actually had the seat would do? Just give up and go and sit somewhere else