When I was 4, i accidentally saw like 5 minutes of "Ghostbusters" and it was the scene where Dana is sitting in armchair and is attacked by some hands coming out of the chair.
For next 2 years i believed there could be monsters inside an armchair which could attack you if you were alone in the room.
I am an adult now and I am still nervous when watching Ghostbusters.
A childhood belief about ghosts

others believed things like this
- as a kid [until age 7] I used to believed that the tv
reporters were literally sitting inside the tv! — nüel - when i was little i believed that if you got three speeding tickets that you went to the electric chairAnon
- When I was a child I used to believe that there were two hands inside the TV making the cartoons move.Anon
- When I was little I used to believe that there was a man sitting inside ATM machines giving you your money.— Kazza
- Atomatic toilets had people inside of them watching to see when you are done.Anon
- I believed you could dry your hands with hot water, until i got a chance to try it.— A A RENN