Question: I got an Error: 0 when registering for an Activision account in Call of Duty: Modern Warfare. What should I do?
Answer: Error: 0 will occur if you attempt to register for an account in game using an email address that is already tied to an existing Activision/Call of Duty account.
Workaround: Back out of the game, then relaunch the game and sign in to the game with the email address that gave you the error.
If you don't remember your password, visit activision.com to retrieve your password.
If you no longer have access to the email address, you will need to create a new account then use that account to sign in to the game.