I recently saw that CanPickUpLoot sometimes won't work. Mobs pick up drops from blocks and mobs, but cannot pick up thrown items.

To reproduce:
1. Spawn a zombie with enabled CanPickUpLoot (or enable it through /entitydata).
2. Drop an iron helmet under zombie's feet.
3. Zombie won't pick up it, even if PickupDelay is manually set to 0.