In the Crown Room of the Delta terminal (#3) at O'Hare airport of Chicago, there is a sign that reads:
All passengers must be on board 10 minutes prior to departure or your seet is subject to reassignment.
As written, it says that I could be on board the plane minding my own business, but because someone else hasn't boarded they can reassign my seat. I used to pick on the Crown Room staff about this every time I came in, but the sign has now been removed (although not necessarily due to my efforts).