I love these threads too. Here are some of my childhood beliefs:
That if I wore shoes without socks, my shoes would walk away by themselves, or that I might take them off and find my toes had vanished. (I think my mum had told me this one, and I was scared at school when we were sometimes made to wear shoes on our bare feet.)
That if things appeared a lot in fairy tales, they must be made up. I was surprised to learn about the reality of wolves, foxes, kings and queens, castles, prisons, rainbows.
When I watched other children being blindfolded to pin the tail on the donkey, I thought they would just see the scarf as a strip in front of them. I couldn't understand why they were all getting their tail in the wrong place! Then it was my turn, and it was a shock to find I couldn't even see the pattern on the scarf. Even after that, I was in denial that the same would happen again: but each time I was blindfolded, I couldn't see a thing. 
@Spanielmadness You must have watched Fawlty Towers. "You're not married, I can't give you a double room. It's the law of England, nothing to do with me."
@hilariousnamehere Although I knew that "moving house" meant the people moved and not the house, when we were told at primary school to write a story about moving house, I wrote about the house actually moving! 