Only time this ever happened to me was when my hubby and I were making a tight connection on Delta through Atlanta. We boarded with just minutes to spare before they closed the doors. Our seats were the last two at the back of the plane. We found another young couple there who had hoped the owners of the seats weren't going to show up. They made a big deal about having to move back to their assigned seats, but no way were we going to let them have the seats for a cross-country flight.

This also reminds me of another situation, not flight related. My parents and I were attending a Niners game that started at 1 pm on a Sunday. My father is a minister, so they couldn't leave until about 11:30 am for the park and it took us forever to get inside, park, get our tix (we had bought them from a friend of mine who works for the Niners), and find our seats. By the time we arrived, it was shortly before halftime and our seats were taken. The people actually asked if we could wait (standing in the aisle) since it was so close to halftime, but we made them get up and move to their seats behind us, since we had bought the seats. Of course, they were loud and drunk and talked about how rude we were the whole time. I've refused to go back to a live football game ever since.