I used to believe that all swimming pools had trap doors to release sharks, like that Bond film. I persuaded myself that I was a prototype human, made by aliens/gods (presumably depending on how many additives I'd ingested). They had made me to check if humans were good or not before making any more. Everyone else on earth was a robot.
The aliens/gods had realised that we were no good from my example and the shark pool was their way of taking out the trash.
A childhood belief about general

others believed things like this
- I used to think that a "pool shark" was an actual shark that lived in a swimming poolAnon
- I used to believe that all of the human beings were Gods little dolls.— Danielle
- i used to believe that pools had underwater doors that sharks were going to swim out of.— marky
- I used to believe that aliens would come and take me if I wasn't good to my parents— somebody
- When I was a kid I used to believe that my perent's were aliens and I was the only human in the house.— Ali
- I used to believe all the stars in the sky were really just aliens with flashlights shining them at Earth.— Lyssy