There are two which immediately come to my mind and both are related to television.
I thought that color (in the real world) didn’t exist until the 70’s because of so many old shows that I watched that were all in black & white.
I thought that all tv shows were filmed in the same location where they were set. So for example I assumed shows like “Different Strokes” and “Barney Miller” were really shot in NYC instead of LA.
Ah the innocence and ignorance of childhood….
I used to think that stock cars were “stop” cars, since when we watched racing on TV, they always seemed to be sliding out of control, like the driver had stomped on their brakes LOL. Probably just the huge overpowered hunks of metal racing on bias ply tires.
Those little packets of raisins:
Well, until I was about 8 or 9 I thought raisins were lollies (candy for my American friends)… Parents although well meaning can be awfully cruel.
Not really a funny idea of mine, but something I may have told my daughters … now I think all of us here are old enough to remember these things
(And if you are not old enough, please keep quiet, I don’t need to feel older 
)
Back to the story, when plugging in one of these for my eldest daughter’s game cube … she asked “what happens if you get the red white and yellow plugs in the wrong holes” being me I glibly replied
“It will blow up”
Fast forward 10 years and she tries to connect a PSP to the big tv … first has to get a av to scart lead… whilst she is plugging it in “ I chime in with don’t get the plugs wrong, it will blow up”
She laughs at me a promptly proceed to connect it up and there was no sound …she changes the plugs round and all is good… she looked at me smugly and said “you lied. It didn’t blow up”
Next morning the wife went to turn in the tv …and nothing …completely dead, now it may have been pure coincidence and the tv was a couple of years old but, I must admit I enjoyed the conversation when the daughter got home
So the moral of the story is if you get the plugs the wrong way round it blows up
We have an old DVD player we occasionally hook up, as well as an older video game console given to my kids. My son once asked the same thing, I told him the sound would play backwards. I meant as in L/R would be reversed, but he took it to mean the movie or video game dialog would literally be reversed. Weeks later, I heard him explain it that way to his sister while watching his mom hook it up again. 
The next time we get it out, I’m going to let him hook it up and then subtly change it to a foreign language, and tell him he must have plugged it in backwards.
I thought once you could see over the top of the refrigerator, you would be an adult.
I thought that in your teens, you went through a metamorphosis of some sort that changed you from the nutburger’s around me at the time, into sane, calm reasonable adults that automatically worked together for the greater good.
I thought girls didn’t like sex, (my christian upbringing) and pretty much had to be cajoled into it. This lasted up to the point they began parachuting out of trees with fire in their eyes and trying to get guys clothes off…
I thought the government was staffed with statesmen (you never hear that word anymore) answering an inner calling to selflessly serve the public, and work together with the other party for the greater good of the nations citizens. (You can tell I did not know enough about adults, yet.)
I believed a lot of stuff.
I think refrigerators got taller, because I still can’t see over the top of ours, but have enough grey hair and achy joints to be firmly into adulthood.