I used to believe that when you flushed a toilet, you had until the water level reached the drain pipe to get out of the bathroom before a terrible monster would come out and hurt me. This resulted in me booking it down our hallway every time I pressed the flush. And we're not even going to touch the automatic toilets in public restrooms.
A childhood belief about fear of

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 a drain pipe in my backyard led to another world.Anon
- I used to beleive that if you flushed the toilet that it would explode and all the water would shoot out.— Michelle
- I thought the "To Let" signs on buildings meant they were public toilets.— Scottish Laddie
- I used to think that in public toilets, the cleaning lady actually lived in the supply cupboard.Anon
- My brother (who was 6 at the time) would never flush the toilet in fear of our new house being blown up! Anon