When I was a kid my mum used to say "sleep tight, hope the lops don't bite" [bedbugs]. For years I thought "the lops" were actually giant fishfingers.
A childhood belief about sleeping

others believed things like this
- I used to beleive if a bug were to bite you, you would become a giant bug.Anon
- I thought my imaginary friend was real and he imagined me
I hope he still believes I existAnon - i thought kangaroos were giant rabbitsAnon
- When I was 8 I thought a spider lived in the toilet and would bite me if I sat on it.— Mike
- when i was young i thought that a giant made the world slowly spin!— ~kassie~
- I thought if someone is in a vegatative state they literally turn into a giant vegetable!— Samilton Duerheiser