Due to the fact that my mother and most other female adults in my life were on the heavier-set side when I was a wee little child, I used to think that their cleavage was actually a scar from when they gave birth and a baby came out of them.
Like, I guess I just didn't comprehend the anatomical shape of breasts, and, instead of seeing two round thingies on people's chests, I just saw it all as one mass of flesh with a divide going down the middle -- a divide that I apparently assumed went all the way down to their bellies (and I blame that last part on adult women bending down to talk to me or whatever and me being able to see down their shirts -- and apparently their cleavage just looked like a massive chasm to me that I thought went on way farther than it actually did). Point is, I thought that was the hole that babies came out of.
Also, I used to think that "phoenix" was spelled as "pheonix" (because that just makes more sense??), and I don't think I even realized that I was wrong until my preteen years. I was very confused by the actual spelling of "phoenix". I'm still confused by it. And every time I see it written out, it still just looks like it should be pronounced "foh-nix", and it always feels wrong to spell it the correct way.
Similarly, I thought that "bury" was pronounced "buhr-y", and I thought it was a slightly different word from "bury" pronounced "bare-y". Not a completely different word, just... a different verb tense or something, I guess. :/ I still never completely grew out of this one, though, and, even though I understand that it's technically wrong, I still can't help but ready "bury" as "buhr-y". That's how it sounds in my head and, sometimes, that's even how I read it out loud. I still say the word normally when I'm not reading it, but... yeah.