So, Hitting A Guardian With Damage Potions Will Crash The Game.

Stack Trace: http://pastebin.com/BEpe5wpZ

What I Think is Happening (Decompiled The Snapshot) Is That The Guardian Thinks It Is Being Hit By Thorns Damage, But That Just My Assumption From The Code: http://pastebin.com/6wsyJN10

You Can See That It Is Trying To Receive Thorns From A Null Entity (Null = Doesn't Exist) (On Line 14 of Pastebin) Cause The Potion Doesn't Contain Who Threw It.
But That's My Assumption.