Until fairly recently an airline ticket was a guarantee of a seat on a flight. It might leave late, but when it did you had a seat on that plane. Some people, however, don't make it to the airport on time. Theoretically, they use statistical regression analysis to analyze tens of thousands of manifests to determine the average number of seats that would normally go to waste on any given flight. In reality, would-be passengers regularly find their seats have been sold to someone else.
Planes sit at the gates while airline personnel try to bribe people to get the hell off.
Hotel and car reservations, which used to guarantee the key to a specific room or a car upon arrival at the registration counter, have become a similarly dicey matter. All the credit-card numbers and confirming phone calls in the world won't protect you from losing your room or your car to someone who flew in at a moment's notice. The high-tech sector is especially notable for its advances in the field of lotto economics. According to a wholly unscientific survey of friends, all cellular providers suck. Like the airlines, they've wildly oversold a service with a finite capacity.