Interpretation of "the earth is round": The surface of the earth is round like a coin (i.e., flat), but the ground in the middle is thicker below the surface (making half a sphere), and the sky is like a dome, making the whole ground-sky combination a sphere. I remember wondering how rocket ships made sure to enter the earth on the right side (the sky, not the ground), since I knew there was no "up" in space.
The international date line causes time travel. If you can stand on one side of it and it's the previous day on the other side, then you could go back in time by going around the world fast enough and crossing it multiple times per day. (When I asked my mom about it, she said something that, looking back, I now suspect is about faster-than-light travel, which didn't help to resolve this misconception.)
Rocket ships regularly have to avoid black holes. They can detect black holes by shining a flashlight in the ship; if they're near a black hole, the beam would have a zig-zag shape, since black holes bend light.
I don't remember for sure, but I think in general I sort of vaguely assumed that we had gotten farther in space travel, and more frequently, than we actually had. I probably wouldn't have been surprised if I'd heard we had achieved interstellar travel. (Also I didn't know we hadn't gotten detailed images of Pluto until not that long before we actually did get a detailed image.)
History and geography
The United States of America started with 13 large colonies covering the entire modern-day US, which then split up into smaller states (like, say, maybe a Pacific Northwest colony split up into Washington, Oregon, and Idaho, or something; I didn't have any specific ideas of what the colonies were), rather than all the colonies being along the east coast and corresponding fairly closely to modern states. [image based on image from Wikimedia Commons; example of the sort of thing I thought, specifics chosen somewhat arbitrarily]
Once someone told me that people from Scotland were called Scots, and I thought they meant that everyone named Scott was from Scotland and everyone from Scotland was named Scott. I don't think I believed them, though.
European colonization of Africa happened earlier than colonization of America, since Europeans knew about Africa first, and I got the impression that Europeans colonized America right after they discovered it, so I assumed the same happened with Africa.
Words and phrases
"Filibuster" is a brand of vacuum cleaner. (It busts filth? Perhaps inspired by the "dust buster"?)
"Rolling one's eyes" meant moving one's pupils in a circular path (i.e., look up, then right, then down, then left, repeat several times; or that in reverse); not "looking up".
"Unemployment rate" = rate at which people are laid off (rather than people who don't have a job). Interpreted as (unemploy)ment, with unemploy as an action (rather than a state); from context I inferred that it was specifically laying off, rather than firing.
"Objects in the mirror are closer than they appear": aside from being generally confusing, I thought this meant that the objects appeared closer in the mirror than they do in real life (i.e., the objects in the mirror are closer than the real objects not in the mirror).
Definitely Disnep, not Gisney or Gisnep. (How do people get a "G" out of that, anyways?) I don't remember if I actually thought that's what the logo said, or just thought it looked like it said that (I knew the company was Disney).
The only reason gay marriage was not allowed was that the people who originally made the law simply weren't aware that people would want it, or they just didn't think about it (i.e., it was an unintentional oversight). The only reason someone would not want to change the law to allow it was general resistance to changing laws; no one would be actively against it. The proposed constitutional amendment (at the time) regarding gay marriage was to allow it.
NSFW/Content warning: talks about private parts (text only, no images)
Each human couple only has sex ("whatever it is people do to have kids") only once (not once per child; just once). That one instance of having sex can result in pregnancy years later, and can result in multiple pregnancies; when and how many times is completely random. That is, once a woman has had sex once, she has the man's genetic material for the rest of her life and can get pregnant with his kid at any time after that until she's too old to have kids (even if she's divorced or widowed and hasn't been around him in years).
Penises aren't involved in sex, only testicles (which I confused with "intestines") are. (Sperm just sort of diffuses through the scrotum, probably.)
Vaginas look the same as buttholes; girls just have a line of three holes down there that look the same, one for babies, one for poop, and one for pee.
"Penis" is a gender-neutral word for wherever pee comes out of, and is derived from the word "pee".
Seventh grade sex ed: How do ketchup and mustard protect you from getting pregnant and getting STDs? Once I realized they were talking about something different, I imagined condoms as like a box that people put their penises into for some reason?
Macintosh/Windows market share was roughly 50/50, with only slightly more Windows computers. (Probably because the main places I had access to computers—home and school—had Macs.)
Scissors and ripping/tearing apply tension force, not shear force. Also scissors removed a line of finite thickness between the two parts of the thing being cut (and I wondered where that went).
When you become an adult, you just somehow …know how to do adult stuff, like having a job and raising children and whatnot.
No-parking signs are actually a crossed-out R (the slash looked like the extra stroke on the R) [image is based on image from Wikipedia]