I know a lot of us got into D&D--whatever edition--when we were pretty young. And I know that many of us, as kids, pretty foolishly misunderstood some fundamental aspect of the game.
I cut my teeth on the Red Box, which only had Lawful, Neutral, and Chaotic as alignments. So when I switched over to Advanced D&D, with its nine alignments, I somehow got it into my head that they were all a single continuum from "most good" (Lawful Good) to "most evil" (Chaotic Evil).
So, for instance, Lawful Evil was still pretty good, and "more good" than Neutral Good. I was seriously confused upon reading Tiamat's entry in the Monster Manual.
I'm curious what other people's goofy rules/concepts misunderstandings were.
I got into D&D during 1e. When I read the books, I misread hit dice and thought hit dice equaled hit points, so a 4 hit die monster had 4 hit points. Two things puzzled me at that point. First, giants and such were REALLY easy to kill for how highly the books ranked them. Second, why the heck did some monsters have 8+4 hit dice when they should have just told me 12?! I figured it out before too long, but for a while my player(it was just me and the upstairs kid) had it really easy.
In the 1E DMG and PHB, if I recall correctly, there are some tables of random things that have 24 items in the list. At the top of the column listing the numbers from 1 to 24, were the helpful instructions "d12, d6", implying that, in order to randomly generate one of the items in the table, one rolled a d12 and a d6. Needless to say, even when I was 11, I could see that the highest number it was possible to generate on those two dice was 18; so I was mystified how to randomly generate a number between 1 and 24 from those two dice.
It was only many, many years later that I overheard someone saying "roll a d12, and then roll a d6, and if the result of the d6 is 4, 5 or 6, add 12 to the d12 roll".
Heh. Every time I encountered a table like that, I'd just figure out the dice to roll to get that spread. D12 + D10 + D4 - 2 and viola! 1-24. The curve got skewed a bit, but we were too young to even realize that. Now days, I still do it. I just don't care if the curve is skewed.
The original Monster Manual didn't have experience point values for monsters killed. That information was in Appendix E of the 1E DMG.
In the far-right column, under "XP Value" it gives the xp base and a variable for how many hitpoints the monster had.
Example: Orc 10+1/hp. So an orc with three hit points would be worth 13 xp.
However, we thought you somehow gained hit points by killing monsters. So whoever dealt the final blow to that orc would get one hit point. If he was wounded, he regained one hp. But if he was already at max hp, his maximum hit points would go up one.
Needless to say, it was like Hackmaster where everyone was jumping around to try to get the final blow on every monster.
When playing 1e, 1” of distance was 10 feet underground and 10 yards above ground. This was his weapon and spell ranges worked. However, I mistakenly thought it applied to spell areas too. We had really big fireballs.
Actually, 1e Fireball was the only spell where the area changed outdoors. Fireballs are very big.
1e PHB Fireball except said:
The burst of the fireball does not expend a considerable amount of pressure, and the burst will generally conform to the shape of the area in which it occurs, thus covering an area equal to its normal spherical volume.[The area which is covered by the fireball is a total volume of roughly 33,000 cubic feet (or yards)]
My friend had received Holmes Basic for Christmas. Being 12 years old we were eager to jump and didn't closely read the book. As a result, we were confused as to whether you were supposed to actually roll the d20 or were to roll the character's class based based hit die. We went with the latter, because it was called the Hit Die. Based on the attack results, we, fairly quickly, realized something was wrong and went back for a closer read, but that was our "Wait, is THAT how that works?" moment.
The relative area of all AOE spells changed outdoors. That's because all distances were given in scale inches, which equaled 10 yards each outdoors and 10 feet each indoors - with the exception of spell AOEs, where each inch was still 10 feet, meaning that their radii (or sides) were effectively divided by 3 outdoors. There's a bit about it on page 39 of the 1e PHB, where Gygax says, only slightly paraphrased: "For God's sake don't forget to divide areas by 3 outdoors!"
Fireball had the additional trait of spreading beyond its normal radius if the volume was too small. It also, as you wrote, mention "cubic yards" as well, so the PHB is slightly inconsistent on the issue.