Totally agree with everyone.
It hasn't happened to me on a flight, but it would very uncomfortable if the offending person was being rude about it !!
On a similar note though..I went to the movies last week with allocated seating. Two people in our seats. I said, oh, sorry, I think you are in our seats. They said.....well someone was in our seats, so we sat here !! They were not moving.
Ok, so we sat in the row in front, and hoped someone didn't come along to us and say.....you are in my seats....haha
Luckily, it was not a full cinema, and no one approached us. However, it can get very messy can't it.
TQ