Tyler Cowen has a few:
I used to believe that it was the wedding ring which somehow caused children to come (really, and yes I was worried about what this meant for traditional scientific theories of causality).
I actually used to believe that you could have a baby by wishing to have a baby, so I always tried really hard as a kid to avoid thinking about babies, which of course, led to my thinking about babies, and then freaking out and parsing whether or not God would think I'd actually made a wish or just had general baby thoughts.
I used to believe I had to eat the crust of the bread because that's where all the vitamins were because that's what my father told me so he didn't have to trim the crusts.
I used to believe that Jim Valvano was the president of the United States because he was on TV so much more than Reagan in North Carolina circa 1983.
And, I used to believe that "argyle" meant "uncomfortable" because I had one really uncomfortable pair of socks that happened to be argyle. So, when my parents told me to put on my argyle socks, I figured they were referring to the overwhelming aggravation factor, not the overwhelming pattern. This misunderstanding culminated in a famous Ham family scene, in which 4-year-old Mary Katharine woke up her father in the middle of the night:
MKH: "Daddy, Daddy. I can't sleep."
Dad: "Why not?"
MKH: "Because my pillow is argyle!"
Dad: "Huh? No, it's not."
MKH: "Yes it is! It's soooo argyle, I can't sleep."
MKH: "Argyle, man, argyle! You're the one who teaches me this stuff!"
Twenty minutes of confusion and frustration ensued before my parents figured out the mistranslation. Argyle still means uncomfortable in the Ham house.
What did you used to believe?