As every frequent flyer knows, a confirmed reservation doesn't necessarily mean a seat on the plane. All major airlines overbook their flights – they keep selling tickets even after a flight is "sold out".

At one time, airlines "bumped" passengers without regard to how urgently they had to get to their destinations. Today, however, DOT (Department of Transportation) rules require airline to first ask for volunteers who, for some compensation, will give their seats. This may be for cash or a free ticket, and usually for a guaranteed seat on the next flight to the traveler's destination.  Only when there aren't enough volunteers is a passenger bumped involuntarily.