When I was young, I used to watch horror movies that came on Saturday afternoons, or occasionally Friday nights if I could stay awake that long. One of them had a woman look into a mirror and and evil witch/demon/whatever looked back at her (can't recall the details). Thirty-something years later, I still refuse to look into a mirror at night because it creeps me out! Oddly enough, though, I never fell prey to the "Bloody Mary" tale, nor do I remember believing there was another world on the other side of my mirror.
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 there is another world on the other side of the mirror, where everyone is named Philip.Anon
- I believed that if I looked too long in the mirror admiring myself the Devil would appear.— Paul Elletson
- I used to believe that my reflection in the mirror was my evil twin that always had bad hair. XDAnon
- I used to believe that there was a person in the mirror who looked exactly like meAnon
- Like most kids growing up, I believed there was an upside-down word on the other side of the mirror.— Taylor (14)