I used to believe that "spare ribs" were extra bones that weren't medically necessary, and could be removed like an appendix or spleen. This belief was bolstered by an urban legend about Marilyn Manson having some lower ribs removed, that I heard later in life.
This belief was debunked, last month, by my fiancee. I'm 29.
A childhood belief about general

others believed things like this
- I used to believe the spleen was an urban legend— Nikki
- My cousin thought ribs at restaurants were people ribs.Anon
- I used to believe that Marilyn Manson and Marilyn Monroe were the same person. — Gina
- I used to believe that when you had a cousin twice-removed, they had died and came back to life twice.— Amanda
- I used to believe that Marilyn Manson was a extremely scary woman!— Tim_NOrway
- I believed the urban legend that gum stayed in your stomach for 7 years if you swallowed it.— Lindsay