The particular rule I'm speaking of is the maximum number of spells per level (one I personally don't use... but then, I also force wizards to learn all their spells outside of leveling up, so wizbooks tend to be much smaller unless they make a ton of money or grab a foe's spellbook). This rule dictates that the most spells a wizard can know in any given spell level is tied inextricably to his intelligence. Once he knows that number, he can never learn any more of the standard D&D spells.
There is no explanation for this. It's a pure balance issue, one designed to prevent wizards from simply knowing every spell there is and becoming ridiculously powerful. Of course, the issue here is that the books don't tell me why in the setting this rule could ever exist. You don't learn a finite number of things in your life and become unable to learn more. No matter what cartoons say, learning new information doesn't push the old stuff out of your brain.
But they could easily have convinced me of this rule. It's a film-making trick that was lampshaded in Thank You for Smoking—fixing it with a line of dialog. If something doesn't make sense, give me a brief explanation as to why it does make sense. That way, if it's important to the film, it can exist without jarring the audience.
So cigarettes in space?
—It's the final frontier, Nick.
But wouldn't they blow up in an all-oxygen environment?
—Probably, But it's an easy fix- one line of dialogue: "Thank God we invented the..." you know, whatever device.
Simple. Almost any inorganic rule can be "hidden" or "converted" in this way into one that is organic. Perhaps magical knowledge takes up physical residence in the brain. Perhaps learning more than that would strain the capacity of the mortal mind and drive it mad! Don't just give me a rule, give me a reason.