"A professor was telling students about his colleagues class. Students in the other class had taken to tossing erasers at the clock. Each precise hit caused it to jump ahead one minute. Before class one morning they succeeded in advancing the clock by ten minutes. Since the new time indicated that the professor was beyond the accepted starting time, the class left. The professor never said a word about the incident. However, he presented the class with a killer of a final exam. As the students labored to finish in the allotted time, the professor amused himself by tossing erasers at the clock."

Source: Interview with Erik Hansen, tompeters.com.