This bug pertains to the over speedy Baby Zombie Pigmen (which probably holds true for the baby zombies too) as well as the incorrect rendering of their swords in their hands. I wont go any further into the speed bug as this has happened back in 1.6 and is pretty self explanatory. However, the sword rendering may need some explaining. Where most pigmen have their swords upright, the baby variant have theirs slightly tilted forward.