There is no six

Started by Golem in humorous top ten lists about Super Mario in which the sixth item would always be absent.

Made a cameo appearance in The Simpsons Movie. (it should also be noted that Arnold Shwarzenegger was World President in Golden Cheesecake Pirate Armada continuity before the movie as well)

Originates, somewhere along the line, from Monty Python's Bruces Sketch, in episode 22, where the faculty rules at the University of Woolloomooloo are:

1. No pooftahs.
2. No member of the faculty is to maltreat the "Abos" in any way whatsoever—if there's anyone watching.
3. No pooftahs.
4. I don't want to catch anyone not drinking in their room after lights out.
5. No pooftahs.
6. There is no... rule six.
7. No pooftahs.