Editing WoW:API UnitGUID (section)

=== Cross-server and GUID "uniqueness" ===
Player GUIDs are local on a per-server basis, making the "global" scope to be bound to the specific server. Every time a character is created, a new GUID is assigned from a simple +1 counter and then given to that character. It should be noted that the act of transferring characters, either server to server, account to account, or even account to account while remaining on the same server will generate a new character GUID, because of how the process works (the character "ceases to exist" for a short period, and is then recreated). This act erases friend and ignore lists. Renaming a character does not trigger a new GUID, as that process is much simpler than a full character move.

Uniqueness is guaranteed in cross-realm battlegrounds by masking the player GUID with a server specific unique identifier, when needed.

NPC GUID collisions have also been observed. It is unknown why or when in specific they occur, but differing mob types have had a NPC ID number which corresponded to an entirely different NPC. This is considered a very rare phenomenon.