So many things...
I didn't understand how lie detectors were supposed to work so I thought you could hook someone up and ask something like "does god exist" and you'd be able to get answers to all of life's big mysteries.
I thought there was a left and right sock
I thought wolverines were mythical creatures
I thought if I tried hard enough I could somehow become older than my older brother like it was just a title or something
Thanks to DARE any time I saw a skittle with the S missing I thought it was drugs even in a newly opened package
I could go on..