I have a rather strange problem to report. I have installed the fglrx module for my AMD HD7750 gfx card, multihead (not Xinerama). When executing a screenlock in XFCE (by default, xscreensaver), and then turning off the monitor, xscreensaver crashes with:
xscreensaver crashed with SIGABRT in __GL_raise()
Which of course unlocks access to the desktop.
This also occured in Debian 7.x on the same machine/same gfx card. If only one monitor is turned off, and the xscreensaver itself doesn't crash, the desktop of the other monitor which is still powered-on will be exposed, although the screen will still be locked for input.
I have also tried this with gnome-screensaver. I haven't yet seen a crash, but I have seen the exposure of the other still-powered-on screen a few times (although not every time).
One solution i've found is to disable OpenGL screensavers either via the 'Settings' button when manually starting xscreensaver (or via the ~/.xscreensaver 'mode blank' config line).
System: Ubuntu 13.10 x64 3.11.0-14-generic) WM: XFCE4 GFX: fglrx (apt-provided: 2:13.101-0ubuntu3) direct rendering: Yes server glx vendor string: ATI server glx version string: 1.4 xscreensaver: 5.15-3ubuntu1 gnomescreensaver: 3.6.1-0ubuntu7
I just wanted to bring this to people's attention, as this could be a security issue.