I used to believe that 'hospital' was pronounced hos-pill-til, because that's where you got medicines and pills to make you better!
I also remember seeing the word 'Mom' written in a book, and thinking that it was a mis-spelling (in the UK we say 'Mum'), not realising that Mom is American.
A childhood belief about speaking

others believed things like this
- When I was young I used to think that WHSmith was all one word, and was thus pronounced "Wwwhhhhuuusmith"— Nathan
- I used to believe that the word 'circumstances' was pronounced 'circus dancers'.— Mel
- When i first saw the word "Czech" somewhere, i pronounced it "siz-ETCH" in my head — Pachisu
- I spent ages wondering what a bart ender was as a child without realising that it was actually bar tender.— Heather
- I remember thinking that non-English speakers were much smarter because they could speak "so much faster." Anon
- I remember thinking that the "Nude Furniture" store was some sort of naughty place. — Texas Rocky