I used to believe that if you were caught alone with a TV that was turned off at night then it would try to manipulate you into "entering" it so that you can entertain others in some sort of magic TV land forever.
I also used to think that any sort of reflective surface I looked into was not reflecting my image, but what I would look like if I was a part of the television's world.
At the end of The Simpsons, there is a short scene after the credits with a bunch of people sitting in a room watching a movie. One of them says, "Shhh!" and then some music plays. This used the scare the HELL out of me. I thought I was watching people who were already trapped in the TV and had basically adapted to their new lives.
When I told my older sister this she marely reinforced these ideas. She also used to turn the volume WAY up when The Simpsons ended, then she would hold me down in front of the TV so that I could watch the people at the end...
I think I was 4 or 5 at the time.
A childhood belief about mirrors

others believed things like this
- I used to believe that the credits at the end of a movie was a list of people in that were watching it.— jon_
- When I was 6 or 7, I believed that people could have yellow skin like in the simpsons— Noah
- I used to believe that the people who "colored" The Simpsons ran out of skin-toned crayons.Anon
- I used to believe that when you turned up the volume on the T.V the people had to shout louder.— Tanana
- I used to believe that someone, somewhere in the world was watching me on their television.— Liz
- I thought Oklahoma was named after my Uncle Homer. (Pre-Simpsons)Anon