It was found that XKB actions for debugging X.org clients were enabled by default. This could cause a screen locking application such as gnome-screensaver to be killed when those key combinations were triggered.

The debugging key actions were introduced in the following commit: http://cgit.freedesktop.org/xorg/xserver/commit/?id=7d2543a3cb3089241982ce4f8984fd723d5312a1

Reference: http://thread.gmane.org/gmane.comp.security.oss.general/6725

Mitigation: http://thread.gmane.org/gmane.comp.security.oss.general/6725/focus=6731