I started with Holmes Basic D&D around age 15 and misunderstood the attack tables. I somehow read "DIE ROLL FOR A MONSTER TO SCORE A HIT, BY DEFENDER'S ARMOR CLASS" to mean "the die roll you need to hit a monster, based on the monster's AC." So the players used the monster attack table to attack monsters, and the monsters used the character attack table to attack characters. The higher the monster's HD, the easier it was to hit it, which made sense to me since most of the high HD monsters were also larger sizes, like ogres, trolls, dragons, giants and purple worms. I figured they were easier to hit because they were bigger targets. Meanwhile the monsters had a tougher time hitting 1st-3rd level characters. I deduced that higher level characters would be easier to hit to maintain game balance since they would be more powerful. My mistake wasn't too much of a problem running B1 In Search of the Unknown because most of the monsters in it only had 1 HD so the players and monsters had roughly equal chances of hitting a given AC. Once we moved into my first homebrew dungeon my beginning players starting chopping down some tough monsters. I specifically remember throwing a hydra at them, and later a couple of manticores. A 2nd level PC would need a 10 to hit the AC 4 manticore, but the manticore would need a 15 to hit the 2nd level PC with the same AC! To make matters worse, those high HD monsters had you roll on the good treasure tables. All that gold meant the players leveled up past 3rd level and out of the Holmes rule set very quickly and they had some good magic items, too! Eventually I read the rules again I realized my mistake, but I let my players keep their xp and loot and soon we moved on to the AD&D rules.