Ah, reminds me of my "impeded fortification" rules from my Mystery Science Theater Monster Project:
Impeded Fortification (Ex): The armor plates of a Metalunan mutant help protect its vulnerable organs, but, ironically, its brain is almost entirely exposed. Critical hits and sneak attacks have a 50% chance of inflicting only normal damage against a mutant, but if they succeed, a critical hit deals an extra multiplier of damage (a greatsword dealing x3 damage, or a pick x5) and a sneak attack deals an extra 2d6 damage.