A small number of Mac Pro owners who have upgraded to Mountain Lion are reporting an issue where the system seems to randomly log out, quitting all applications without any warnings and returning to the log-in screen.
While this may appear as if the system has restarted or even performed a standard log-out, the system is in fact experiencing a crash in a critical process called the Window Server that supports the current log-in session by managing the drawing and representation of windows on the screen. Crash logs that affected users have revealed show that the problem is rooted in the way certain graphics functions are being handled by the window server, since specific actions of the system's CoreGraphics framework are referenced in conjunction to the crash.
This problem has been happening for affected users quite regularly since upgrading to Mountain Lion, and may be rooted in some advanced desktop management features such as Expose, Missions Control, or switching between multiple desktops.
It has become apparent that this error is only happening for people who have multiple monitors attached to more than one GPU installed in their Mac Pro systems. In testing, those who have switched to using just one video card either by unplugging monitors from the second card or removing it from the system have found when doing so the problem disappears. In addition, those who have booted the system in Safe Mode (which only loads a minimum driver set and disables graphics acceleration features), have found the problem does not persist in this mode. Performing a clean installation of OS X by formatting the hard drive and reinstalling has shown no change in behavior, indicating that a software configuration issue is not to blame.
These suggest the problem is likely a driver fault; however, it could also be a bug in the Window Server that occurs when performing certain actions that used advanced graphics capabilities.
For now, if you own a Mac Pro and are affected by this issue, then you can try unplugging the monitors from all but one video card, or remove extra video cards from your system. While this will cut down on the number of monitors you can run with the system, it will prevent interruptions in your workflow. If you have a multi-GPU Mac Pro and are thinking about upgrading to Mountain Lion, then you might consider waiting for now to see if this issue is cleared in the first updates to Mountain Lion.