here's one (true story):
a guy speeds and he pulls over when the cops signal him down. after going through the standard procedure the cops give him his ticket and walk back to their car. the guy asks the cops one more question: "excuse me, officers? can you help me here? my pant leg is really itchy around here..."
the cops find a bag of cocaine...
as for stupid signs? there should be two types: "gullible" and "too ****in' stupid to breed". the gullible people are understandable. they try to follow the rules, and when there's an ambiguity they assume the worst. they certainly look stupid to the plain observer, but what can you do?
then there are the "too stupid to breed". these are the people who turn their head ever so slightly at intelligent behavior and mutter a "ehuugh?" they make it clear that any attempt at conversation will be met some quantity of stupidity (normally a buttload, but sometimes a donkeyload). these people can be found in horsefly hatcheries, pea-soup factories, and Capitol Hill.