As a kid I believed that the word "artificial" was actually "art official", as if artificial things were products of some kind of "official" art.
A childhood belief about spelling

others believed things like this
- I used to think that prostitutes were female lawyers. It just sounded so official...— MC
- i used to think the word "friendship" meant a special kind of rocketship that you went on with your friends.— chips ahoy!
- I used to believe that people who were lactose intolerant could not consume ANY dairy products AT ALL. — Marideth
- I used to believe thato to adopt a child you had to collect points from supermarket products...— Linda
- I thought that serial killers had serial numbers like products did so the cops could identify them easier. — C
- my husband believed, until recently, that the word "several" meant seven of something. — sheryl