when spawning a mass quantity of zombies if you punch one of them they will all mass together and stutter (game tick lag). there shows signs of block, mob, and redstone lag however there are no signs of FPS drops or player movement lag.
video proof : http://youtu.be/dMs3ca7lNdM
(video may not me up if so then try again later)
Duplicate of MC-14683, please use the search function to see if your bug has already been submitted. Currently over 55% of tickets are being closed as duplicate.