When I was about 4, I saw this ad on TV for a promotion in which you could win a Gameboy (I think it had something to do with Breakaway bars) which showed a kid playing on one and ending up with holes in him (not sure of the specifics, but they were round holes you could see through, not bloody gunshot wounds or anything).
For a year or two after that, I thought that would really happen to me if I played on a Gameboy, and I was scared that someone would buy me one for Christmas or my birthday.
A childhood belief about adverts

others believed things like this
- I used to believe, the little round bandaids, were to cover bullett holes.Anon
- Me and my sister used to dig holes in the yard because we thought we could dig to china.— Savannah
- I use to believe I had holes in my bum cheeks for my farts to come out— jackL
- when i was little i believed that leprecauns put the holes in crumpets.Anon
- I used to think that the moon and stars were holes in the sky that you could see heaven through.Anon
- i believed that the holes in swiss cheese were cow farts.— lily