We have recently discovered that possibly all screensavers with lock capabilities are prone to a cleaning lady attack. By issuing Alt+SysRq+f you (or a malicious cleaning lady) can make the OOM killer kill your screensaver process and give you access to the screen. This works with xscreensaver, xlock and gnome-screensaver (Debian Bug). As a hotfix, you can write
# echo 446 > /proc/sys/kernel/sysrq
to disable killing of processes with Alt+SysRq+f.