When using Alt-Tab to switch between tasks on my Ubuntu box, I occaisonally encounter a horrible flicker. This flicker very visibly alternates a fullscreen window between it's content and a black screen. Occasionally this would hang my PC, prompting a reboot. Othertimes it would seem to reset my video card (requiring an X restart).
The workaround is provided by the Ubuntu bug tracker, in Rui Boon's comments on bug #155053, which suggest tweaking an option in compiz. The pre-requisite is to install compiz-settings-manager, if you haven't already:
sudo apt-get install compizconfig-settings-manager
Next, run Compiz Config Settings Manager (which might be known as "Advanced Desktop Effects Settings"), from the System/Preferences menu.
Find and uncheck the option: Unredirect Fullscreen Windows, in the General option's General tab.
Close the settings manager, and you should be flicker-free.