Ever since reading Douglas Hofstadter's Metamagical Themas, I've been fascinated with the theme of self reference. Gödel, Escher, Magritte, Lisp, self-stultification, the Epimenides Paradox (with implications on trust), Buddhism -- it comes up in many forms.
This sign displays it in a humorous way.
Other humorous examples of self reference can be found in Hofstadter's books. I've spent late nights cracking up with friends reading parts of Metamagical Themas aloud. Okay, maybe I'm easily amused by internal dialogue. Here are some other examples (from this site):
Break every rule.
All generalizations are misleading.
If somebody loves you, love them back unconditionally.