After hearing a comedy/parody show where the superheroes referred to their nemeses as "evil fie-ends" I honestly believed for YEARS that the word "fiend" was pronounced "Fie-end". It was highly embarassing when at age 12 my friend pointed out that I'd been mispronouncing it.
A childhood belief about speaking

others believed things like this
- I used to think that 'black comedy' was show like Family Matters and The Cosby Show.— Katie
- 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
- When I was a child I used to believe that my parents WERE superheroes at night.
Anon - When I was a child, I used to believe my parents were superheroes, like the fairly oddparents!Anon