On a recently upgraded AMD64 Debian Etch system (nVidia Quadro FX 3400) xscreensaver started freezing the xserver on some of its random screensaver transitions. Over time I believe I have traced it to a faulty xscreensaver configuration file (xscreensaver-demo/advanced menu) which now has "Text Manipulation" defaulting to an unspecified "Text file" instead of the previous "Host Name and Time". After changing the selection to the latter the problem seems to have gone away. I believe the non-existent file selection was causing xscreensaver to do something strange when it would eventually try to start a screensaver which required text input, and I've notified JWZ about it. 

But as JWZ rightly points out, no matter what xscreensaver does, it shouldn't be able to crash the xserver, and it seems to be doing that. Once it is frozen, the graphics freezes, it doesn't respond to keyboard or to mouse. The system is still working fine underneath, because I can ssh in from other machines. If I kill xscreensaver it does not release the xserver. If I kill gdm, it will not restart. Nothing short of rebooting the machine will bring back the xserver (that I have found). 

Jeff Cunningham