Many 20th century high-rise buildings do not have a thirteenth floor. The simple reason is the superstition that the number thirteen brings bad luck. As there are people who will not rent or stay in a room on a thirteenth floor, that floor is 'skipped'. The knowledge of buildings not having a thirteenth floor helps to perpetuate the superstition, providing an excellent example of recursion.

"A fear of the number 13 costs America over a billion dollars a year in absenteeism, train and plane cancellations, and reduced commerce on the 13th of the month." --Superstitions, by Suzanne Lord