I used to believe that you should never ever turn on the faucet in the bathroom sink right after flushing the toilet -- because the water going down the toilet would come out of the faucet! A similar belief held for drinking fountains located right outside of public restrooms.
A childhood belief about general

others believed things like this
- 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 you didnt get up before flushing the toilet would eat you...Anon
- i used to think when it started raining it was god flushing the toilet— edel
- I was certain that rain was created by God having a pee and flushing his huge toilet above us— Emma H
- I used to believe that if you didn't close the toilet lid after flushing, you would get sucked down into it.— Maya
- My best friend used to believe that when it rain and thundered that God was flushing his toilet. — Jelly Bean