When a friend of mine moved to the US from India, he thought that the 'Haz Mat' signs on trucks carrying hazardous materials meant that you could not laugh if such a truck passed by, or while looking at the truck (Huz - to laugh, Mut - not, in Hindi).
Then I pointed out to him the seeming double negative in the 'No Haz Mat' warning signs on roads leading to tunnels, etc...
A childhood belief about road signs

others believed things like this
- I wanted to grow up and be a truck. Not a truck driver, a truck. Anon
- I used to believe the Do Not Pass signs on the roads meant you were'nt aloud to pass the sign. I was 14.Anon
- i used to believe that the signs for curvy roads were for snake crossingsAnon
- Having seen enough red octagonal STOP signs in my time, I used to look out for green circular GO signs.— Gary
- I thought the "To Let" signs on buildings meant they were public toilets.— Scottish Laddie
- Up until I was 17 I thought that 'no outlet' signs meant that the steet didn't have any streetlights.— Kelsey