As I mentioned above, I've had this crash when not running Compiz (only one though), so it seems more likely to be a bug with X, but one that is made much more common by Compiz (and usually by simultaneous intensive disk IO and CPU).
As I mentioned above, I've had this crash when not running Compiz (only one though), so it seems more likely to be a bug with X, but one that is made much more common by Compiz (and usually by simultaneous intensive disk IO and CPU).