I used to never be able go to the bathroom unless the shower curtain was wide open. I thought that people could use the shower drain as a portal to my bathroom, and that they might be trying to spy on me. Now that I think about it, it was really quite sad, and I still have to pee with the curtain open (even though I'm 14 now), but from looking at this site, it appears to be a common childhood fear.
A childhood belief about bathrooms

others believed things like this
- in this TV show i saw a snake coming up from the drain so everytime i take a shower i go really fast!— scared person
- I used to think that trolls lived in my shower drain and showered with my dirty water— Troll bather
- I used to believe that take a shower meant to go steal a shower from the storeAnon
- I used to think that toilets led to the shower head. That's why I never took a shower :D— scared
- I used to believe that if you took a shower after a day of tanning the tan would come off in the shower— Shar
- I thought you could die from chewing gum while in the shower.— Hollow Head