Open Radar

 
I had one MacBook signed into iCloud. I used Migration Assistant to clone the old machine onto a new machine. When the new machine asked me to sign into iCloud it accepted my password but after entering my 2FA code presented a activity indicator for hours before throwing an "unexpected error". I then skipped iCloud setup so the Mac would start-up and tried to log into iCloud from System Preferences. The same error occurred. It then dawned on me that maybe iCloud was confused since the "old" computer was still logged in. I logged out of iCloud on the old machine and was then able to log into iCloud on the new machine to continue my setup.

Resolution: This kind of problem should alert the user with a descriptive error and instructions for resolving the conflict if there is one. Less knowledgable users are likely to give up and start setting up their new machine causing iCloud sync issues down the line or create a new AppleID.