There's a huge thread (over 800 comments) on AskMetafilter about this question:
This mistake is surprisingly common:
Definition of 'erstwhile' - I thought it meant strong and steadfast, instead it means former. (link)In senior year of high school, I had a logophilic friend who gave me a copy of his yearbook photo. On the back of the photo, he wrote a nice note with a pen and signed it: "Your erstwhile friend, [his name]." When I pointed out the correct definition of the word to him, he added a superscript "1" after "erstwhile," then wrote a footnote at the bottom: "1. And current!"
Here are some more language-related realizations:
I was in my teens when I realized that those "hors d'oeuvres" that I read about and those "ordervs" I heard people talk about were one and the same. (link)
I thought Chanukah and Hanukkah were two different holidays, and that I'd just never heard the first one spoken since I'd never heard anyone say CHanukah. One day I was in line at the grocery store where I'd shopped for years. The person in line in front of me had a loaf of challah bread on the belt. The cashier said "Man! Isn't our HALLA bread just the best?!? And I immediately said to myself "OMG Chanukah is Hanukah!" I was 25. (link)
I didn't know what "penultimate" meant until recently. I thought it was a synonym for "ultimate". Oh the shame when a friend corrected me. (link)
A few years back I learned that the proper spelling of dilemma does not have a silent n. Up until that moment I was 100% stake-my-life-on-it certain it was spelled dilemna. (link)
Macrame and macabre are unrelated words. (link)
I was nearly 30 before I realized that my parents had been talking a Wodehousian language during my youth and that normal people didn't refer to each other as "old bean" or "the mammal" or share a refreshing snort before dinner. (link)
I was about 25 when I figured out that Soup Du Jour wasn't actually a certain style of soup. (link)
I was probably 12 before I realized that people on the radio singing to "my baby" weren't singing to their infants. (link)
NEXT: More things gotten wrong.