Because of Beauty and the Beast, I thought that "provincial" meant "boring" because she sings, "I want much more than this provincial life!".
It also made me think "primeval" meant "prime evil", e.g really bad, because Belle uses that word to describe Gaston.
At 31 I moved from Italy to London, UK. I used to believe that the phrase "out of spit" was a particularly graphic and rather dramatic way of expressing one's animosity and vengeful intententions. Only 8 years later, after moving back to Italy, I eventually realized that one is supposed to say "out of spite".
There's a website called TV Tropes that has a trope called "Heroic BSoD", which means a hero has an emotional breakdown.
Because of this, I thought BSoD was a disorder, like OCD or PTSD, that meant you were prone to emotional breakdowns.
Later, I learnt that it's actually an abbreviation for Blue Screen of Death.
I thought for a long time that the word "awry" was pronounced like "aw-ree" (rhymes with sorry) , instead of "uh-rye". The weird part? I had heard people say it lots of times, but it took ages before I realized it was the same word
I used to think you could go to jail just by saying a word wrong during a speech.
I used to think that weasel was short for wooden easel, and was confused as to why someone would be called a piece of art equipment as an insult.
I thought "prison" and "prism" are the same words as each other.
I used to think that retarded was the same word as retired.
I used to think that if something was out of the question, then you were banned from asking for it.
When I was 15, I accidentally referred to my baby cousin's Christening as his funeral. Well, at least I found it funny myself.
I used to think headquarters meant somebody's head cut into 4.
I thought the phrase "throw the towel in" had to do with throwing one's swimming towel into the pool/ocean/lake/river.
Stereotypically, I used to think that all Indians used the phrase 'oh my God' in just about every sentence.
Until one of my parents actually used the word out loud, I used to see the word 'deluxe' on my pet rescue game, and thought it was pronounced 'DELCKS-chur'.
I thought that polystyrene was pronounced 'polly-STEE-LEE-an'.
I used to think allegations were made by alligators.
I was little and saw a movie that involved a girl falling into a river. A man exclaimed, "The current is taking her!" and me, being five or six and never having heard of currents, thought he was saying "currant" like blackcurrant.
When I was five or six, I was watching TV (I think it was "The Simpsons") and a character mentioned a piece of paper but I misheard and thought she (I'm pretty sure it was a woman) said, "piece of *vapour*".
When I was younger, I thought the word vinegar was the same as the word vagina, and in year 5, we had a TV crew come to our school, to do a documentary on school dinners, and I said into the microphone, which was turned up quite loud 'well I usually have ketchup or mayonnaise on my fries, but my older brother, in high school, prefers vagina with his'. Needless to say, that part was obviously deleted.
I thought that "Oh, the humanity!" was an exclamation meant for pointing out that something was very humanlike.