Items held in the hands of zombies do not render.
Steps to reproduce:
1. Open a world in 14w05a (any previous version should work).
2. Spawn lots of zombies in an enclosed area.
3. Throw a bunch of non-weapon items in to make the zombie pick up the items.
4. Fly away so all zombies not holding an item despawn.
5. Open the world in 14w05b.
6. Throw a wooden sword at one of the remaining zombies. The sword should disappear.
7. Throw a stone sword at the same zombie. The stone sword should disappear and a wooden sword will appear, as the zombie switches to the better weapon.