When I was four, I thought cars told drivers where to go when they got lost.
My grandpa was driving and said "I'm lost." Then, I saw a green arrow flashing in the car, he turned in that direction, and then he said "oh, now I know where I am."
(I thought the car told him where to go. Later, I found out that drivers put the signals on to tell other driver's where they are going.)
The cashier at Quick Chek told me (when I was like 4) that I'm a "pretty little girl" and asked me what my name was. I told her that my name was Wilma. (Because I thought everyone knew what I knew. I was Wilma, I called my grandpa Fred, my grandma, Betty, my uncle, Barney, my mom's other brother, Dino, my aunt Kris Barbie, because there weren't anymore characters.) The cashier gave a dirty look to my mom because she actually thought that my mom named me Wilma. lol Then my mom said "come on, Ashley, let's go."
Another misconception I had as a kid was that everybody of all ages watched Barney and only when you're in first grade, you can ride in a wagon. I also believed that people thought it was bad thing for a girl to like boys. I've secretly (and later on publicly) liked boys since forever, but couldn't let my friends know, because they'd make fun of me and say that I have cooties. I knew it was okay to like boys once I was in middle school, because that's when people started dating. Also, I used to think that if a girl talked to a boy, then that means everyone thought that they were boyfriend and girlfriend.