Mac OS X 10.4.6 or later: Scheduled shut down occurs at Login window even if users are logged in
In Mac OS X 10.4.6 or later, if you have scheduled a shut down time in Energy Saver preferences, your Mac will shut down automatically at the scheduled time while at the login window, even if other users are logged in via fast user switching. This happens without any warning, which means if the logged-in accounts have files open that have unsaved changes, the unsaved data will be lost.
Note: If the computer is not at the Login window, scheduled shut down will still occur, but an alert will appear if any users have unsaved changes.
To avoid the possibility of losing unsaved changes when automatically shutting down from the Login window, do not leave the computer idle at the Login window if user accounts are logged in with unsaved work.
Tip: If you want to "lock the screen" while you're away from the computer, use the Security preferences option "Require password to wake this computer from sleep or screen saver," then start the screen saver or put the computer to sleep before leaving the computer idle.