I used to believe (I was 6-7) that when you flushed the toilet you had to run out of the bathroom, spin around three times, look at a corner in the room and be very quiet because a tiny demon who lived in the toilet would otherwise get you for disturbing his sleep... I still can remember the expressions on my parents' faces :)
A childhood belief about fear of

others believed things like this
- I used to believe that monsters lived in my toilet and they ate my poop and urine when i flushed the toilet.Anon
- I used to believe that if I flushed the toilet before I showered, I would take a shower in dirty toilet water.— Chris D
- I used to think if I left the toilet seat open my cat would get flushed down the toilet.— G
- i used to be scared to use the bathroom becuase i believed something lived in the toilet.— robert allen freeman the fourth
- I used to believe that the toilet would suck me down if I flushed it.— :-)
- i use to believe that if i flushed the toilet when i was still on it would suck me down to who knows whereAnon