Growing up in America, I always knew gherkins as pickles. When I was around 12 I heard the word gherkin, and I thought it was a sex toy or a dirty word of some sort. I didn't find out otherwise until very recently, I'm ashamed to say.
A childhood belief about gherkins

others believed things like this
- In first grade, I thought that if I've never heard of a word before, then it wasn't a word.— Destiny
- up until recently I thought intern was just another word for a secretary (I'm 27 btw)— Axecutioner
- my husband believed, until recently, that the word "several" meant seven of something. — sheryl
- Like most kids growing up, I believed there was an upside-down word on the other side of the mirror.— Taylor (14)
- When I was young, I heard the word 'minion' for the first time and thought it was a miniature onion.— Karin
- For years I heard the word "pedophile" and thought it meant a pedicurist's tool.— Lauren