I used to strongly suspect that every mirror was actually a tw-way mirror. This was easy to check at home, and I was pretty sure the house bathroom was okay since it faced an outside wall, and I checked closely to make sure there weren't any holes or anything like that.
Public restrooms on the other hand...
A childhood belief about mirrors

others believed things like this
- when i was young i used to believe that my image on a mirror would come to life if i broke the mirror.Anon
- I used to think that if a mirror with me in it breaks, I break.Anon
- I used to believe that there was a person in the mirror who looked exactly like meAnon
- I used to believe that when you cracked a mirror your reflection would come and take over your body!— XxXAbbeyXxX
- I used to think that if I looked in a mirror for over 5 mins I would dissapere to Narnia!— Quako
- I used to believe that my reflection in the mirror was my evil twin that always had bad hair. XDAnon