When I was a kid starting out gaming, I didn't have a lot of resources to turn to. No internet, no discussion groups, not a lot of fellow gamers. So some things I just didn't get, or understand, or wholly misunderstood. Here are a few I remember.
Men are mean. It's true. The Monster Manual says so. Under intelligence, it says stuff like "Mean: average to very." I figured, okay, they're average to very intelligent, but mean. Fair enough.
Knowing the "mean" in terms of "average" is probably still not something most 10 year olds know. Or maybe they do, now, but no one corrected me back in the day. And it never occurred to me that this was odd enough to go look the word up.
So, men are mean. My NPCs are still, on average, mean.
Night Hag Rides. It was decades after I first read Night Hags (also in the MM) that I stumbled across the phrase "hag ridden" and realized it had a connection to sleep paralysis. The whole odd actually being on the person's ethereal back thing is still a little odd to me. But as a kid, I just thought, what the heck? And moved on. We never, ever used Night Hags or their special power if they happened to show up in an adventure. It was confusing, we didn't get the reference, and thus it was left aside.
"1 of each magic excluding potions & scrolls." Oops. Good thing so few monsters have treasure type U or V. Because I read that as one roll on each table. Yes. On each table. One on rings, one on rods/staves/wands, one on weapons, one on artifacts, one on each of the miscellaneous magic item sub-tables. Every. Single. One. That's 11 items.
I just assumed that those monsters were supposed to have great hordes of magical treasures, 70% of the time. The other 30%? Nothing. Like I said, good thing those monsters were rare. And that we generally ran modules.
Lucern Hammers are for clerics. What? It says it's a hammer. We ignored most of the odd polearms because we didn't know what they were, but this was clearly a hammer. We were victims of the "put in everything Oakshotte mentions" syndrome without knowing it until we hit those illustrations in the original Dungeon Masters Adventure Log. Suddenly, no more hammers doing 2-8 instead of 2-5.
There is other stuff I didn't "get" but which are so oddly written nobody seems to have gotten - treasure types being for wilderness only (er, why is that?), the helmet rule (you can choose hit locations?), how initiative worked (we thought we got it, but we didn't), encumbrance, etc. But my total misunderstandings? The ones I remember are above.