I was about 5 years old. My mom’s friend’s husband was apparently very good at cultivating houseplants. My mom said he had a “green thumb.” Next time we visited I was sitting on the man’s knee looking at his hands trying to figure out why she’d say that when clearly neither of his thumbs were green.
I watched the news with my parents all the time. When the newscaster said that someone had been “raped”, I thought they were saying “raked” and pictured someone attacking with a garden rake. For some reason I also thought that “slain” meant that the victim died from the attack with the rake.
When I was a kid, I wondered why the other kids referred to their sneakers as “tenna-shoes.”
I don’t know whether this counts as a mondegreen or not, but I was confused by the song “She’ll Be Comin’ Round the Mountain.” I didn’t understand why someone coming around a mountain should be an occasion for people to go outside to the gas meter. As in, “We’ll all go out to meter when she comes.”
Watching Wimbledon tennis on the TV, I thought ‘deuce’ was when the players sat down for a drink between games.
I recall not understanding the difference between “prosecuted” and “executed”.
It made “NO Trespassing - Violators will be prosecuted” signs a little terrifying.
When I was a teen, I was asked to do a point/counterpoint on Euthanasia. I had no idea what was so special about the kids in the Orient.
As a small kid watching football on a crummy 50’s TV, I was very confused when they said it was “1st Down and Golden Glow”. It still looked like the other plays to me.
When my parents talked about my uncle “getting carried away”, I mentally pictured a bunch of guys picking him up (on a blanket, maybe, like those fire-rescue nets) and Carrying Him Away. Later on, they would “drop him off”. My uncle was always getting “dropped off” somewhere.
In kindergarten, I thought the phrase “all of a sudden” was “all of a sun.” I knew what the phrase meant, but I thought it was “all of a sun.”
Eh, nevermind, mine really didn’t fit the criteria.
Reminds me! My dad grew up in, and my grandma lived in a little town called “Orient”. Confused the hell out of me when I would hear people refer to “The Orient”.
It wasn’t really a case of mistaken hearing but when I was little I thought if you got fired, they just locked the office door and set the room on fire.
I also thought that getting drunk was something you could only do 2 or 3 times before you ran out of brain cells. It made it slightly weird when I started reading YA/A books and characters would talk about getting blitzed for fun. “Oh no,” I’d think, “Every character in this book is suicidal!”
A classmate in grade school, Peter, had “progressive” parents who treated him like an adult. He would bring his dad’s porn to school, things like that.
One day the teacher was out of the room and the kid behind Peter repeatedly hit the back of his head with a pencil. Peter eventually spun around, just as the teacher returned to the room. Everyone shut up and Peter, unaware, screamed, “Cut it out, you DOUCHEBAG!!”
The teacher, a sweet older woman, freaked the fuck out, dragging Peter from his seat and off to the principal’s office. We were freaked out as well. A bunch of 6 or 7 year olds trying to figure out what Peter said and trying to find the word in our pictionaries.
My dad told me once when I was three or four that if he didn’t show up for work, he’d be fired. I was terrified that his employers would come after him and have him whacked.
When I was five or six, I was reading a Little Lulu comic book (the humor and writing in these were actually quite sophisticated) in which someone used the word “misled.” For years afterward, I thought it was pronounced “MY-zeld,” and meant “screwed,” as in “I was screwed out of my share.”
In grade school some of us were a little (harmlessly) rowdy during a field trip. The next day the principal called an assembly to yell at everyone for making the school look bad. He said that if any of us ever acted like that again he would suspend us. I thought that he meant that he would whip us with a pair of suspenders and I went home and told my parents that he had threatened to beat us.
I don’t know why I thought that because I was never beaten as a child and, as far as I know, no one at that school ever beat anybody.
My last year in college, I was taking an advanced class in 20th century US foreign policy. The girl who usually sat next to me once handed in a paper about fighting Communist gorillas in Vietnam.
Her comment when she got the paper back with lots of red pencil marks all over it? “Well, SpellCheck said everything was okay.” Duh! :smack:
Humorous aside: For the rest of the class, I was doodling Communist gorillas on a piece of scrap paper and sliding it over to show her. When the instructor saw what I had been doing, he laughed out loud! 
When I was little I asked my grandmother why she wasn’t answering the phone. I thought she said “because it isn’t Irene.” I, of course wondered how she could possibly know who it was (caller ID wasn’t even a wild idea yet, at least not publically.) When I was a little older, and found out how party lines worked, I realized she said “it isn’t our ring.”
In junior high I got in trouble for calling a kid a dildo. I thought it solely meant a foolish person. I started using the dictionary after that incident as opposed to gleaning meaning from context.
I used to think “acoma” was a word, and didn’t realize it was “a coma”.
I also used to have a dilemma when medical news came back as “negative”, and it turned out to be a good thing. Of course, the flip-side of that was that I thought “positive” results were good…
Conversely, I used to wonder what a “stigmatism” was, and confused it with a “stigma.”