I don't know if this counts as a misconception necessarily, but when I was younger I though that the "rear view mirror" was actually called the "review mirror". I think that's because I grew up in Boston though :D.
Lol, my civics teacher in sixth grade was from eastern Massachusetts. The entire class thought that King John had issued the Mag-nah Cah-tah in 1215. I can still hear that intoned in a perfect Boston accent, even more so than the "pahked the cah in Hahvahd yahd" phrase.
I used to think that you only had sex once... and then you had however many kids you were going to have and thats why some people have 8 kids and others had 3 and some people only had 1. I just couldn't imagine intentionally wanting to have 8 kids.
An even funnier one was that for a short while I didn't think girls did number 2. I put women on a pedestal. They were different, Purer, Cleaner, then guys... How could they possibly do something as disgusting as 'poop'?
LOL Fortunately it didn't take long for the right girls to come around and rub my nose in reality!!
When I was about 3 or 4, I thought everything I saw on TV was done by little people inside the TV set.
Like the others, I thought Jesus and his father were real & like the boogie man and Santa Claus, they watched over me and would punish me if I was bad. Took me a while to crawl out from under that paranoia.....
When I was 12, I thought I would be too young to be the first human on Mars (in 1972 during the last Apollo moonlanding, Apollo 17). At that time, NASA was still talking (or had recently finished talking) about manned Mars missions by 1986.