When I was super young going through the McDonalds drive through, I thought that they made the Happy Meal toys in the store in some kind of giant Santa's workshop-type factory, like, right before they give it to you. Imagine my disappointment when I found out they just get them imported in
I would say "having a bawl" instead of "having a ball", and "compensation" instead of "conversation"
I saw an anime vid of a character putting on a cowbell necklace and then turning into a cow, and being as stupid as I was back then I commented "There's no way this can happen in real life though, right?" and someone (probably being sarcastic) replied "If you wear a cowbell then you turn into a cow, or if you are male then you turn into a bull. Everyone know that, it's common knowledge." and I actually believed it... I even went on the Wikipedia page for cowbells and everything to see if there has ever been any record of... a human turning into a cow.... Gosh I was stupid..
When I was super, super young (like, toddler age) I didn't know that "drowning" was just losing air while underwater. What did I think it was instead? I thought that somehow, you get shrunk down and get sucked down the drain of the bathtub/pool. I was horrified of swimming and didn't even learn how to until a late age...
Also when I was about the same age as the drowning fear, when going down escalator stairs, towards the end I was horrified of keeping my feet on the moving stairs because I thought that I'd get vacuumed down or something?
I thought that the moon had oceans on it. Because our 3rd grade teacher tried to teach me about how the moon affects the ocean's tides or whatever, and I completely misinterpreted that..
I thought that anything before the year 1900 was prehistoric dinosaur times...
I miscategorised a lot of animals. I thought that raccoons and skunks were rodents. I thought rabbits were rodents (they're not, they're lagomorphs, but I remember being taught in school that they're rodents because SCHOOL LIES), I thought newts and salamanders were reptiles, ughh I got fish and aquatic mammals so confused, like I thought whales were fish and that sharks were mammals.. I thought possums and opossums were the same thing.. I thought ponies were baby horses... and so on..