I don't recall believing anything that I've since outgrown. However, I couldn't believe that pronunciations could differ. It was a great concern of mine when I was nine and we were to move from Tennessee to Illinois. I remember repeating "bean" over and over and never believing that it could be pronounced other than the way I's always heard it. Big surprise!
A childhood belief about speaking

others believed things like this
- My mom had me believing that, like fruits and vegetables, the crusts of bread were the healthiest part.— StephTheGeek
- I thought my age started at 4, because I couldn't recall any mention of my age before.Anon
- I grew up believing meerkats have human hands because I learned about meerkats from The Lion King.Anon
- Because of Pocahontas, I grew up believing Native Americans noises sounded like English to Native Americans.Anon
- When I was little, I used to believe that Illinois (where I lived) was a countryAnon
- I can remember thinking that is I hid my eyes, and couldn't see anybody that they couldn't see me.Anon