There are a few I can think of off the top of my head:
The first thing I believed for the longest time was that unless all passengers in a car or other vehicle were wearing their seat belts, the car simply would not start, not even if the key was in the ignition. This was probably because in my earliest years (starting when I was a newborn baby thru most of my childhood) my mom and dad had a rule that until everyone had their seat belts on, the car didn't move. If I were to balk at wearing my seat belt (though I'm told I never really did) then too bad; we weren't going anywhere until I thought better of it and strapped myself in/allowed myself to be strapped in. Imagine my surprise some years later when I would go places with friends during playdates and the adult(s) present would start moving the car before/during the seat belt buckling process!
I think I was about 8 years old when I finally learned that unbuckled seat belts and fully operational cars were not mutually exclusive.
....In retrospect, I think part of the reason I didn't mind wearing a seat belt (aside from the fact that I learned right away that it was a non-negotiable rule, and was actually very rule-oriented as a kid) was because I liked the tactile sensory input of the seat belt criss-crossing my body - I found it soothing. However, I didn't like it when the seat belt got stuck, but then, who does?
.....When my parents took me to the mall to see Santa for the first time when I was maybe three, we happened to be walking past his picture-taking station on our way out (I was angling to get to talk to him again), and he wasn't there. I asked my parents where he could have gone, and my mom told me something like, "He probably went up on the roof of the mall to go feed his reindeer - they must be hungry!" And I accepted it b/c hey, I was three.
They were certainly lucky that it didn't occur to me to ask if we could try to look for him on the roof once we got outside so I could tell him goodbye.
....Thanks to
Who Framed Roger Rabbit and the interstitial in
Fantasia where Mickey Mouse "approaches" conductor Leopold Stokowsky to congratulate him, I thought that animated characters really existed and could interact with human beings....a belief that has over time evolved into more of a fond wish than anything else.
When you've felt alone for most of your life, even in the company of other humans, such a longing never really goes away.
...I also believed that bran flakes/cereal were peanuts for some reason.
I'd always ask my mom for a handful of them whenever she made bran muffins so I could leave them out on the kitchen table...I was under the impression that Dumbo (the Disney elephant) would pass through and eat them, but only if no one else was around.