Some of my favorites:
"When you see a worthy man, endeavor to emulate him. When you see an unworthy man look inside yourself."
Confucius
"It ain't what a man knows that get him into trouble. It's what he thinks he knows, but just ain't so."
Will Rogers.
People tend to live up to, or down to, our own expectations.
Anon.
"When there are only two people in an elevator and one farts, it is difficult for the one who did it to convince the other they did it did. Some are fool enough to try. Some are fools who try to prove they did not, and give them credence."
Itchy