I ran into an interesting situation a few years back going out to LV. I was meeting a group of girls who had all met on the internet. I was traveling with two other girls who lived in the DC area (I live in DE), so we all met up at BWI.
We booked the three seats together in the front of the coach aisle, because they were both very tall. I mean very tall. As we were waiting to board, the gate agent called us up to the podium. They asked us if we would mind trading our seats with a nice newlywed couple that couldn't get seated together. We said absolutely not, we booked those seats weeks before for a reason, because of their height. The gate attendant said, no problem.
Lo and behold, we get into the plane and they are our first two seats, the window and the center seats. The aisle and window seat behind us were vacant. They just wanted to move up one row to get the leg room and sit next to each other (instead of asking the middle seat to switch). We were arguing with them as they were sitting in our seats, and they said, no, the gate agent told us we could move to these seats. Since we were the last row to board, we are now surrounded by flight attendants who were about to throw US off the plan for refusing to sit in our seats......while they were allowed to remain seated in ours. The flight attendant said if we did not sit in our seats and stop arguing with the nice young couple, we would be thrown off and had to book another flight at our own expense. I was almost in tears. Thank goodness, Nia kept her cool and she very nicely showed our seat assignments to the one flight attendant who decided to listen to us.
She asked the couple what made them think they could take any seat they wanted, and they said the gate attendant told them they could have them. The flight attendant told them it was an outright lie, because the gate attendants are not allowed to re-assign seats unless they are available. She apologized to us, and told the newlywed couple they had ten seconds to move to their original seats or would be removed from the plane....
You should know that 
