When I was younger a friend of my mother told me that her car wouldn't start until all the seatbelts were fastened. I believed this for a good few years, and was mocked mercilessly when I was found out.
A childhood belief about cars

others believed things like this
- I used to think that the moon was fastened with something so it wouldn't fall— Kelvin
- I believed if I prayed for the Devil, he would start being good for God. — Jackie
- My Mother used to tell me only the good die young and I would live to be 100+ years old because I was bad.— William
- my mother told me if i ever swallowed chewing gum i'd turn into a pig, i believed this for many years.— stuart hayward
- I used to believe that I could fly like Superman if I had the right cape and got a good running start. — Rick
- My mother told me when i was 5 that Santa wouldn't come if i kept sucking my thumb. — Traumatized