User blog comment:TheNewBuildGuy/Possibilities-- Jungle Zyra/@comment-5266525-20130227111309/@comment-5266525-20130227213754

You cannot penetrate any more magic resistance or armor than the target has, or in other words you can't do increased damage with magic penetration alone. You're talking about magic resistance reduction, which is impossible to obtain with just runes and masteries and is generally very expensive to get ingame (talking and  here).