This is actually my college roommates belief.
You know how wheels on cars sometimes look like they are going backwards when the car is going fast? Well, her father told her that they really were going backwards, because everyone knows that for every action, there is an equal and opposite reaction.
Well, when she was 16, she was out with friends, and someone mentioned how funny it was that wheels look like they go backwards, and she proceeded to enlighten them with her father's explanation, that she had believed for all those years. :o)
A childhood belief about cars

others believed things like this
- I once believed that one could run around the south pole in order to travel backwards and forwards in time.— kaibutsu
- i used to think that french was english backwards!— ---pepsi---
- I used to think that Russian was just English backwards— Millie
- I used to believe the D in the Disney logo was a backwards G. I never understood why.— Nancy
- I used to think my priest was a bit foolish for never having noticed that Dog is God written backwards— anticap
- I used to believe that if I spelled Norwegian words backwards, it would be English. I don't anymore...— Gudrun