Overbooking is a common practice of airlines in which the company will deliberately assign more passengers to a plane than can actually fly on it, based on the assumption that some passengers will cancel flights. This ensures that planes are nearly always full and thus maximizes profits for the airlines. But perhaps this practice needs to end, because of the following incident:
http://www.cnn.com/2017/04/10/travel/passenger-removed-united-flight-trnd/