My birthday is September 1.
For 20 years, I believed that that was 9 months after New Year's Eve, and that I was just a giant mistake after too much drinking.
When I got my PhD in math (really), I was able to figure out that in fact I was born 9 months after Thanksgiving!
A childhood belief about reproduction

others believed things like this
- I used to believe babies were 9 months old when they were bornAnon
- My sister used to think that because I was 15 months older than her, I would die 15 months before her!— Nicola Braden
- Until about 6 months ago (I'm 21), I believed a bidet was used to wash your feet in.
Eep!— Rebekah - My parents have the same birthday -- month, day, and year. I though all mom's and dad's had the same birthday.— Cait
- I used to believe that people born on leap year would only age once every 4 yearsAnon
- I used to think once you got pregnant, you couldn't have sex at all during the 9 months.Anon