it happens from time to time I dont know why (I would assume stress/survival tactic) but if you can get the head to sit still under decent lighting in medium/low current it will continue to develop and grow a new skeleton. being in a LFS kinda lends more to my theory of stress/survival. but that my take on the scenario. I know the heads can survive and I have heard of it happening many times, so I would call it semi normal.