(This one isn't even from childhood, I was about 17 for this...) There's a doughnut shop I go to which sells "sour cream doughnuts" - that is, doughnuts with a little sour cream added to the batter to make it thicker. But apparently no one ever explained this to me; because at some point, after years of frequenting that doughnut shop, I had roughly this conversation with my mom:
ME: Oh no - I think they put sour cream doughnut holes in with the rest of ours!
MOM: So just don't eat them.
ME: But...they're all in the same box. Won't the sour cream rub off on the others?
MOM: ...
That's right - I'd been seeing those doughnuts in the display case and assuming that the standard white sugar glaze on them was, in fact, a light coating of sour cream. For years. I never questioned the idea that people would actually want to eat something like that - I've always been a picky eater, so I just assumed it was one of those food things I didn't "get".
A childhood belief about sweeties

others believed things like this
- The first time I saw fog, I thought the air had turned sour.— jamby
- when i was 4 or 5 i used to think neon sour gummy worms were real because they always twisted and turned— HUGBUG09
- "Yooce cream, ice bean, we all scream for ice cream!"
This song was always stupid to me.— Shirley B. - My parents told me that when the ice cream van was playing music, it means he's run out of ice cream :(Anon
- My dad told me that ice cream vans only play music when they have run out of ice cream. Anon
- My dad would tell me that if the ice cream van was playing a tune it meant that he had no ice cream left. Duh!Anon