Active Directory users are unable to log in using the Mac login Window.
The screen will shake as if an incorrect password has been entered.
Enable the option in System preferences:
1. Go to the following location on the MAC:
Apple Menu | System preferences | Users & Groups (or 'Accounts' if using OS X 10.5/10.6)
2. Unlock the menu by clicking on the padlock icon
3. Click on 'Login Options'
4. Select "Allow network users to log in at login window"
5. Click on 'Options' and make sure that the 'All network users' option is selected.