Dumbest Common Denominator

The most logically-challenged portion of the general population for which the most obvious warnings, posts, signs and instructions must be written or designed to avoid catastrophe.

On the lid of the coffee cup: Contents may be hot.
On a package of kitchen knives: Caution! Items are sharp.
On a trash bag: Do not place over head.
At a railroad crossing: No parking.
On a clothes dryer: Do not place children or animals inside.

"This is the dumbest sign I've ever seen. Do they think I'm a complete moron?"

"Dude. Gotta plan for the Dumbest Common Denominator."