My Mac runs 10.6.6 and has a machine account on the Active Directory at work. Today at work, I was in System Preferences->Accounts->Login Options and, by complete accident, I unchecked “Allow network users to log in at login window”. What ensued was a waste of time, so I hope this saves you time, should the same happen to you (I posted a bug report with Apple today and I hope it is resolved in 10.6.7 or Lion).

After unchecking the box, the spinning beachball of death immediately came up and I was forced to restart my unresponsive MacBook Pro. When it came back up, I couldn’t log in to any of the accounts on the MBP, admin or not, network or local, even the root account. I freaked out and tried to reset my passwords by booting the Snow Leopard DVD and then I reinstalled Snow Leopard, but none of this helped!

Eventually, I happened across Brian Keefer’s solution.

To summarize his post, fix things by booting your Mac to the Login Window and in the username field, enter “>Console” without the quotes and press enter. Your screen will go dark for a few moments and then you’ll be prompted for your username and password. Enter credentials that use to work and you should be able to successfully authenticate. Now fix the problem, by using the line below with your own username and password substituted:

dseditgroup -o edit -a localaccounts -u yourusername -P yourpassword -T group com.apple.access_loginwindow

Repeat the command above for any other user accounts that you are not able to log in with. Now type in “reboot” and you should be able to log in as you use to.