I used to believe that in war movies, the soldiers of the bad side (for whichever war was the subject) were played by criminals, and the US side used live ammunition.
A childhood belief about criminals

others believed things like this
- I used to believe that if I chewed on one side of my mouth too much the other side would get jealous.Anon
- When a soldiers went AWOL in the movies, I thought they physically climbed over "A Wall".— D Rumsfeld
- I was convinced that Gorilla soldiers in Vietnam in the 70's were real Gorillas attacking other Soldiers.— Mario
- i used to believe that everyone had a twin on the other side of the world.— tahnee
- I used 2 think that the scar on my side said "UNUSEABLE" LOL!!!!!!— John In Pennsylvania
- Until I was 23 I used to think the ginger put with sushi was a side of ham.— maari