To reproduce the crashing of xserver one should:
1. use XScreenSaverSetAttributes() to set attributes for xscreensaver
2. use XScreenSaverUnsetAttributes() to unset previously set attributes
3. use XCloseDisplay to close the connection to display

When program comes to the point where display should be closed, it's 
xserver will exit with signal 11.

Here's the source of xcrash program which should crash X server upon
execution.

/* 
 * Compilation: 
 * gcc -g -O2 -Wall -Wstrict-prototypes -o xcrash xcrash.c \ 
 * -I/usr/X11R6/include -L/usr/X11R6/lib -lX11 -lXext -lXss 
 */ 
 
#include <stdio.h> 
#include <stdlib.h> 
#include <unistd.h> 
#include <getopt.h> 
#include <X11/Xlib.h> 
#include <X11/extensions/scrnsaver.h> 
 
#define PROGNAME "xcrash" 
 
 
int main (int argc, char *argv[]) 
{ 
char *disp = NULL; 
int parameter = 0; 
Display *display; 
Visual *visual; 
Colormap colormap; 
int screen; 
int screen_width; 
int screen_height; 
int depth; 
Window root_window; 
GC gc; 
int event_base; 
int error_base; 
XScreenSaverInfo *ssinfo = NULL; 
XSetWindowAttributes attributes; 
 
 
/* Getting arguments using getopt_long */ 
while (1) 
      { 
      int option_index = 0; 
      static struct option long_options[] = { 
                                            {"display", 1, 0, 'd'}, 
                                            {0, 0, 0, 0} 
                                            }; 
 
      parameter = getopt_long (argc, argv, "d:", long_options, &option_index); 
      if (parameter == -1) 
         break; 
 
      if  (parameter == 'd') 
          disp = optarg; 
      } 
 
if (optind < argc)
   { 
   exit (EXIT_FAILURE); 
   } 
 
 
/* Connecting to a server */ 
if ((display = XOpenDisplay (disp)) == NULL) 
   { 
   fprintf (stderr, "%s: cannot connect to X server %s\n", PROGNAME,
XDisplayName (disp)); 
   exit (EXIT_FAILURE); 
   } 
 
/* Let's use sync mode for debugging purposes. */ 
XSynchronize (display, True); 
 
 
/* Using macros provided by Xlib to obtain info about display */ 
screen = DefaultScreen (display); 
screen_width = DisplayWidth (display, screen); 
screen_height = DisplayHeight (display, screen); 
root_window = DefaultRootWindow (display); 
depth = DefaultDepth (display, screen); 
visual = DefaultVisual (display, screen); 
colormap = XDefaultColormap (display, screen); 
gc = DefaultGC (display, screen); 
 
 
/* Testing the existence of Xscreensaver extension */ 
if (XScreenSaverQueryExtension (display, &event_base, &error_base) == False) 
   { 
   fprintf (stderr, "%s: XScreenSaverQueryExtension - screensaver extension not
supported\n", PROGNAME); 
   exit (EXIT_FAILURE); 
   } 
 
/* Allocating an XScreenSaverInfo structure */ 
if ((ssinfo = XScreenSaverAllocInfo()) == NULL) 
   { 
   fprintf (stderr, "%s: XScreenSaverAllocInfo - insufficient memory\n", PROGNAME); 
   exit (EXIT_FAILURE); 
   } 
 
/* 
 * If some other client already used XScreenSaverSetAttributes we should not 
 * try to do the same. It means that external screensaver will be used and 
 * ssinfo->kind will be 2. 
 */ 
if (XScreenSaverQueryInfo (display, root_window, ssinfo) == 0) 
   { 
   fprintf (stderr, "%s: XScreenSaverQueryInfo - extension not supported\n",
PROGNAME); 
   exit (EXIT_FAILURE); 
   } 
 
if (ssinfo->kind == 2)
   { 
   fprintf (stderr, "%s: some other client has already set screensaver
attributes.\n", PROGNAME); 
   exit (EXIT_FAILURE); 
   } 
 
/* Some window (screensaver) attributes */ 
attributes.background_pixel = BlackPixel (display, screen); 
attributes.border_pixel = BlackPixel (display, screen); 
 
 
/* Normally, we would use window with override_redirect attribute set. */ 
XScreenSaverSetAttributes (display, root_window, 0, 0, screen_width, 
                           screen_height, 0, depth, InputOutput, visual, 
                           CWBackPixel | CWBorderPixel, &attributes); 
 
if (XScreenSaverQueryInfo (display, root_window, ssinfo) == 0) 
   { 
   fprintf (stderr, "%s: XScreenSaverQueryInfo - extension not supported\n",
PROGNAME); 
   exit (EXIT_FAILURE); 
   } 
 
XScreenSaverUnsetAttributes (display, root_window); 
 
fprintf (stderr, "To dye or not to dye... It is not question any more.\n"); 
 
XCloseDisplay (display); 
 
exit (EXIT_SUCCESS); 
}


I have reported this problem to xfree86 team and they have already
solved this problem.
Some friends of mine reported the same problem with xorg-6.7.0 so,
I decided to report this bug to xorg, too.

Thanks in advance.