Iron golem attacks monsters but when they are done, they are still stuck in "attack-mode" : They keep waving their arms, moreover, sometimes:
-when you place a block before them they jump without stopping.
-they keep walking forward while waving their arms
They return normal after finding another mob to kill.
Duplicate of MC-590, please use the search function to see if your bug has already been submitted. Currently over 51% of tickets are being closed as duplicate.