There are times after my system goes to a GL screensaver and PowerManager is set to turn the display off soon after lock the screen, I cannot get any keyboard or mouse input to work to turn the screen back on and ask me for a login password.
The Caps-Lock key no longer lights up on my laptop in this state. Previously I thought the system was frozen. However, after some research from a different system over SSH, I found that the screensaver was running at 100% CPU load on one of the cores. I killed the PID of the screensaver, and keyboard input returned which allowed me to wake the screen and get a KDE unlock window to return to my Desktop. The Caps-Lock key lights up again if that matters to you on how the system is responding to input.
Most users likely hard reset the system. This is not necessary if you can SSH back in. This is also why I could not find a similar bug since the rest of the bugs I saw were about the system itself freezing.
X.Org X Server 1.11.3
Release Date: 2011-12-16
Description: Ubuntu 12.04.1 LTS
Release: 12.04
FGLRX: atiInstaller-8.951-12.30
KDE: KDE Development Platform: 4.9.00
This will not reproduce the error, but the current xsreensaver I used was flocks, which you can see with:
'kxsrun flocks -- -window-id %w'
I thought to possibly create a workaround by providing a keyboard shortcut that would kill the screensaver process, however I am unsure whether is would create a security issue or if it would work at all.