I have had people in my seats many times. I even had someone start getting surly with me at a MN United game because they told me that they had pretty much the whole row and we were going to have to move. I asked them if they had seats 18-21? Yes, we have pretty much the whole row, you need to move. Row 12? Yes, the whole row. You need to move. Section 4? This is section 5. No lady section 5 is over there. She then stormed off mad because I was right.
Usually it is a combination of wrong seat number, wrong row number or wrong section when there is confusion about seats.