Install KDE workstation with graphical login. At the kdm dialog, shutdown. The X server flickers a bit, and then dumps out EIP and stack and stuff telling you to report it to the XFree86 people. If no-one can reproduce it I can try to catch the messages if you like. This is 100% reproducible on the two machines here in the office I've been testing on.
With 6.2beta3 this is a bit better -- it actually shuts down and reboots when told to -- but the X server still crashes. I'm using a Matrox G400. (With gdm on a GNOME workstation I don't see this)
This only seems to happen with a Matrox Millennium G400, so I guess it's an XFree86 problem.
please report to XFree86 directly.