User blog comment:Khnx/League of Math Ep.2 - All about Hentai (Penetration Charts)/@comment-9728354-20140830200624/@comment-24766419-20140830201630

It isn't. Flat PENTRATION cannot bring effective armor below 0. Only flat REDUCTION can do that (which is immensly rare btw).

http://leagueoflegends.wikia.com/wiki/Armor_Reduction#Flat_armor_penetration