Rifts is a game with a lot of strange rules. Some are just old. Some are poorly explained. Some really don't make sense. One part of the rules that is probably mocked more than any other is the concept of Mega-Damage.
Mega-Damage first appeared (to my knowledge) in Palladium's licensed Robotech line of RPGs. The idea was that regular damage couldn't do justice to the scale of destruction that the giant mecha of the animated series were capable of dealing out. Enter Mega-Damage, one point of which was equal to one hundred points of regular damage (or "Structural Damage" in Kevin Siembieda's parlance), but with the caveat that attacks that dealt regular damage would never have any measurable effect on a Mega-Damage structure.
Siembieda's argument was that nothing that wasn't heavily armored could survive a direct hit from a tank cannon. Furthermore, you would never be able to inflict any serious damage to that heavily armored tank with something that wasn't specifically designed to do so. No matter how much of a badass you were, you could beat on that tank with a baseball bat all day, but you'd never do much more than scuff the paintjob. You could spray the tank with an Uzi and get similar results. But break out an anti-tank weapon, and you might be getting somewhere.
Mega-Damage, as I've mentioned before, is a much-lampooned concept, but I don't really understand why. (I think it must be the name, which is admittedly a bit goofy.) I think it makes a lot of sense. I certainly think that it makes a lot more sense than the extremely abstract concepts of "hits" (which aren't always hits), "damage" (which isn't always damage), or "healing" (which isn't always healing) in Dungeons & Dragons, a game which hundreds of thousands of people still play and enjoy without apparent confusion. At any rate, objections to Mega-Damage were apparently common enough that Siembieda provided guidelines on using removing Mega-Damage from the game in the Rifts Conversion Book, fairly early in the line's long life. We never used them.
Still, there are areas where the concept of Mega-Damage breaks down, or has what I think were probably unintended consequences. For one thing, the widespread availability of Mega-Damage weaponry in Rifts means that characters tend to walk around in full environmental body armor at all times. Even tools like laser torches used for welding can inflict Mega-Damage, which means that a street thug with a dinky laser pistol has the ability to level a city block or wipe out your lovingly crafted character with a single hit. For another, by the rules as written, even a glancing hit from a Mega-Damage weapon will almost certainly kill your character instantly -- you're either armored and okay, or you're dead and turned to a fine mist. A good GM can work around these issues, but they remain nevertheless. (There was an attempt to address the "either you're okay or you're bloody mist" problem in Rifts Ultimate Edition, but its so-called "GI Joe Rule", where you always survive the hit that depletes your armor, but are now unprotected and presumably running for your life, actually is the cartoonish joke Mega-Damage has been made out to be, and has just made matters worse.)
For the upcoming Rifts campaign in which I'm participating, my GM is experimenting with reviving an older Palladium system, Armor Rating, which by the book works similar to D&D's Armor Class: if you roll over the Armor Rating, you've bypassed the armor and damaged the person inside. That seems potentially promising, but I'm not sure how it will work for something like a fully enclosed vehicle (like the vast majority of the mecha in Rifts) or for living creatures that are Mega-Damage structures, which are plentiful in the game. I know that he's using AR in a different way, however, and I'm interested to see what he does with it.