I used to believe that Orson Welles (the filmmaker) had also written the famous books "1984" and "Animal Farm", which recently I found out were in fact written by George Orwell (the actual author). And "by found out" I mean just ten minutes ago.
A childhood belief about films

others believed things like this
- I used to believe that a vitamin was a small animal
only recently I have found out that it is not true... :)— Sylwia - I used to believe that the Bible was written by Moses, with God whispering everything in his ear, in English. — Jj
- I used to think my priest was a bit foolish for never having noticed that Dog is God written backwards— anticap
- I used to think that 'non toxic' was a color. I always did wander why is was written on every crayon.— Maddy
- I though the "D" written in "Walt Disney" on some videos was actually a backward "G".
— A. Nonymous - I used to believe anon was a person and couldn't believe he had written so many poems— Helena