When I was little, i thought a "loan shark" was a literal shark. I remember hearing my parents talking about loan sharks and thought they were going to get bitten by a shark if they didn't pay any money.
When I was a kid I thought tasers make people too sleepy to do anything
When i first heard someone talk about "ripping somebody off", i thought it meant they literally ripped the person's skin off their body.
I used to think the sound of a heavy church bell was meant to be the chime of doom, and that God was ringing it himself as a ghost. Grim but still funny.
Judging by the sound of the word, I used to think an asylum was where everyone had to be silent. Well, it is a type of prison!
For some reason, I used to think it was illegal to make up stories in your head, without writing them down on paper!
I used to believe that copying definitions out of a book counted as plagiarism!
I used to believe that police officers were basically the entire criminal justice system. I thought they could arrest anyone at any time and send them to prison for as long as they wanted.
When I was about 5 or 6, a police officer visited my school to give a talk. I asked a question and called him 'Sir' but he told me to call him 'Dave', not 'Sir.
I immediately burst in to tears because I thought I had angered him and he was going to send me to prison for the rest of my life.
My mother has this friend whose last name is Stalker, so when I heard people talking about stalkers, I thought they meant my mom's friend and people related to her.