We've gotten a few reports recently about issues with the Name Tags. Some players have had problems when they try to swap their pets names by using name tags. For example, you have a pet named "Joe" and you want to name another of your pets that. So you use a Name Tag to change Joe's name and another name tag to give the name "Joe" to another one of your pets.
Now, most of the time this will work. Some people have reported issues where it says the name of the pet is still taken. Let me explain why this is.
There was a bug that would allow users to hatch pets with a duplicate name. It wasn't checking the names of the pets in the graveyard for a long time. So, you might have gotten a good name that wasn't really available. You got a duplicated name.
What happened is, the script was fixed and now it verifies for duplicate names in the grave yard. So pets you hatched with a name may be a duplicate. When you try to swap names, the game will now recognize that it was a duplicate and give you an error message.
So, be careful when you are using a name tag in this fashion. The name of your pet may be a duplicate.