double negation
Curious notice at the entrance of my local supermarket...

The text says: "No entrance for (high school) students between 12:15 and 1:15 PM." (Not a very convincing statement from a legal perspective I'd say, but that's not the point.) Take a look at the icon they designed for it.

The icon communicates exactly the opposite of what is intended. It says: "Prohibited to keep people out." A perfect visual example of a double negation.