I watched an episode of Blue Peter at my Grandparents' house, and on it was a supposed "mummified mermaid."
It scared the heck out of me, and from then on, whenever I used the toilet I would be terrified that one of these demonic-looking mermaids would rise out of the toilet bowl and bite me on the bum.
I had to look down all the time, and be ready to jump up at any given moment, in case one apeared.
It took a long time to get over that.
A childhood belief about fear of

others believed things like this
- I used to believe that if I sat on the toilet too long a snake would come up and bite me.— Cheesemonkey
- When I was little I thought that if I stayed a long time in the water I would become a mermaid.— Salma
- My brother (who was 6 at the time) would never flush the toilet in fear of our new house being blown up! Anon
- I used to believe that toilet seats were for kids only and that adults sat directly on the bowl. — Evan Klyne
- I used to believe that if you jump in a toilet, they lead you to another dimension.— beep.
- I used to believe that crocodiles lived in the toilet and would bite my butt when I had to go to the loo.— Oz