In 15w34d your memory usage goes very high mainly between 30% and 50% and crashes alot, also whenever you leave the world minecraft crashes, also I was making a mob arena and when I spawned mobs in they wouldn't attack I was in gamemode 0 and they wouldn't even charge they would just stand I gave them weapons also they don't use shields and other arrows with a bow or sword also you can change a locked difficulty(I don't know when that came in) with /difficulty.