I used to believe that if I slept with any piece of bare skin showing from under the duvet I would die. I don't remember how I realised this was wrong, but evidently it was.
Also, my cousin told me that I would be killed by a ghost if I was still awake past midnight.
A childhood belief about ghosts

others believed things like this
- I used to believe that showing my bare arms meant that I was naked, so I never wore short sleeves as a kid. Anon
- I used to be scared that if I chewed my cuticles and swallowed a piece of skin, I would become a cannibal. Anon
- i used to believe that stars really were star shaped, i realised at the age of 34 how wrong i was.Anon
- I used to think that dreams were events that really happened, I just couldn't remember them when I was awake.— Warren
- I loved watching the cartoon Casper the Friendly Ghost and wondered why the Holy Ghost never got his own show.— Robert
- I thought the Holy Ghost was the ghost of the Virgin Mary.— Texas granny