The rules at a particular university were such that if the
professor was not present in the classroom by 15 minutes past
the hour, the class was considered a "walk" and the students
were free to leave — with no penalties for missing a class.
The rooms were equipped with the type of wall clocks which
"jumped" ahead each minute, in a very noticeable fashion. As it
were, these clocks were not of the most sophisticated
construction. Some enterprising student discovered that if one
were to hit the clock with chalkboard erasers, it would cause
the clock to "jump" ahead 1 minute.
So, it became almost daily practice for these students to take
target practice at the clock (this particular professor was not
the most punctual, and the students considered him severely
"absent-minded"). A few well-aimed erasers, and lo, 15 minutes
were passed, and class dismissed itself.
Well, when the day for the next exam rolled around, the
professor strolled into the room, passed out the exams, and told
the class, "You have one hour to complete the examination".
The professor then proceeded to collect the erasers from around
the room, gleefully took aim at the clock. When he had
successfully "jumped" the clock forward one hour, he ended the
class and collected the exam papers.