The one floor rule
The understood rule that unless you are handicapped and cannot walk, you never take the elevator when only going one floor up or down. Failure to comply by this rule results in wasted time for all other elevator passengers, and will undoubtedly lead to death stares and possible bodily harm to said offender.
I was late for my big meeting because some idiot was too lazy to take the stairs and decided to break the one floor rule.