In the Installing Active Directory Part 2 video of the Working with Active Directory course, (around 1.30-2.00 mins) the trainer alludes to the fact that local logins do not work once a server is promoted to a Domain Controller by typing in dc01\administrator as the User ID (which eventually fails).
However, I believe local logins will still work on a Domain controller e.g. in this case if we simply login using Administrator (no prefix) we should be logged in using the local Administrator account and not a Domain account.