Minecraft wiki entry for Name_Tag, associated text: Name tags were added at the request of Paulsoaresjr.[2][3]
A stack of up to 64 name tags can be renamed at once. The cost is 1 experience level per stack, regardless of how many name tags were stacked.
To name a mob “Name Tag” the player must give the name tag a random name, then rename it back to “Name Tag”.
A villager with a name tag turned into a zombie villager by a zombie with a name tag does not despawn, but a villager with a name tag turned into a zombie by a zombie without a name tag does despawn.
It is impossible to have a rainbow sheep upside-down, because it is impossible for it to be named “Jeb_” and “Dinnerbone” at the same time.