I have two moments from Pokemon Silver.
When you meet your rival in the game, he says "My name is ???!", the ??? meaning you have to give him your own name. Later, Professor Elm asks what his name was. I actually thought that ??? WAS his name, so I entered "???"...
Also, I named my character Max. Later in the game, I found a max potion, and I was absolutely convinced that the potion that restores your pokemon to full health is always named after what you named your character.
A childhood belief about toys

others believed things like this
- I used to believe the the tooth fairy was a Pokemon fan because she got me Pokemon Crystal for my lost tooth— Pokemon fan
- when i was growing up i had two imaginary friends. One was named Jennifer, the other was named Onion.— Tessica
- I used to believe there was a Russian Leader named Garbage Trough and an Arabian named Are A FatAnon
- The boy in my class named "Sahand" must have a sister named "Safoot".Anon
- I used to believe that the game The Legend of Zelda: Twilight Princess was the longest game ever.Anon
- I thought Oklahoma was named after my Uncle Homer. (Pre-Simpsons)Anon