give a mob or a player a player skull with like so
/replaceitem entity @e[r=3] slot.armor.head skull 1 3

everything goes weird. things go upsidedown and on other locations.
hurt the thing wearing the hat and your screen turns red or you crash.

video:http://youtu.be/VFccMP1VzHc

fix with: /replaceitem entity @e slot.armor.head air
and punch him again