Originally Posted by Ribido
I don't think that magic penetration should take a person into negitive resistance for reasons I will list below.
Magic Penetration does NOT work beyond 0 magic resistance.
Minus Magic Resistance Debuffs
% Based Magic Penetration
Linear (Flat) Magic Penetration.
If Magic Resistance <= 0, Magic Penetration doesn't take any more effect.
Additionally, if the % based works like Last Whisper (did? Not sure if this was fixed), then if the total resistance is below 0 (reduced by methods listed below), then having the %-based penetration actually reduces your damage instead of increasing it due to the % not accounting for negatives, it takes a % regardless.
With items like Banshee's Veil giving 60 Magic Resist, and champions having an inherent 30, it's likely a 20 point of magic resist will never hit 0 unless you have an Amumu, Evelynn, Lich, Soraka, and spec'ed Exhaust all on the same team with an abyssal sceptre nearby as well
(The only current % Based Magic Penetration is in the form of a Mastery in the Offensive tree.)