Contingent behavior exists when each person's actions depend on what he expects others to do. Its role is obvious in our day-to-day behavior. The way a person going to a party dresses will depend on how he expects others to dress. People select the time and place they eat lunch on expectations of when and where others will eat. The route a person takes to work depends on what routes he expects others to use. Students decide whether or not to cut a class before a vacation based on how many other students they expect to cut the class, and the amount they study for a test is often based on how much they expect other people to study.
The existence of unintended consequences in contingent behavior is illustrated in the pictures below. In both pictures, the behavior of the people involved depends on how they see others acting. You should be able to see that in the first picture below the end result will be that two people keep hats on.