I thought that Roman Numerals were pronounced like the letters making up the numeral rather than like the number. For one birthday (I think I was about 8 or 9) I got the game Mega Man IV and would keep referring to it as "Mega Man I.V" (pronounced just like the abbreviation for "intravenous") even though my parents corrected me. I'm not sure how long it took for me to say it right.
A childhood belief about counting

others believed things like this
- Even though I knew about Roman numerals when I was younger, I thought that there was 11 world wars.Anon
- When I was a kid I believed that the number 11 was pronounced "onety-one"— Alicia
- i thought the name Anon, was really a name...pronounced A-Non...haha i figured it out— Norman einstien
- Until very recently, I thought "archives" was pronounced like "are chives".— Mitsuki
- I thought that polystyrene was pronounced 'polly-STEE-LEE-an'.— Polly Stairiin
- When I was little i thought lb for pounds was pronounced lubs, and i called them that for 2 years. Anon