I was adding my own hacks into wmcpu dock applet (which runs at size 64x64).
I made a little typo in the code and Xorg segfaulted.

wmcpu has this kind of code:

#define copy_xpm_area(x, y, w, h, dx, dy) \
    XCopyArea(display, pixmap, pixmap, gc, x, y, w, h, dx, dy)

My error was with param1:
copy_xpm_area(3, 75, param1, 9, xoffset + param1, 5);
when it went too big, Xorg segfaulted.

If you want specific test case, I think I can might bother to do that in a
couple of days.
But meanwhile, here were the logs:

Fatal server error:
Caught signal 11.  Server aborting

X connection to :0.0 broken (explicit kill or server shutdown).
XIO:  fatal IO error 104 (Connection reset by peer) on X server ":0.0"
      after 129790 requests (129773 known processed) with 0 events remaining.
/usr/bin/xinit:  connection to X server lost.
X connection to :0.0 broken (explicit kill or server shutdown).

...

Fatal server error:
Caught signal 11.  Server aborting

/usr/bin/xinit:  connection to X server lost.
/usr/local/bin/wmaker warning: got signal 1 - exiting...

xterm:  fatal IO error 104 (Connection reset by peer) or KillClient on X server
":0.0"
xscreensaver: SIGHUP received: restarting...
xscreensaver: running as safari/safari (500/500)

...

when values are "not too fscked to cause segfault",  I get:

/usr/local/bin/wmaker warning: internal X error: RenderBadPicture (invalid
Picture parameter)
        Request code: 152
        Request minor code: 7
        Resource ID: 0x4015b2
        Error serial: 93933



I run fedora's xorg-x11-server-Xorg-0.99.3-9 -i386

(--) PCI:*(1:0:0) Matrox Graphics, Inc. MGA G200 AGP rev 1, Mem @ 0xd8000000/24,
0xd4000000/14, 0xd5000000/23
...
(II) Module mga: vendor="X.Org Foundation"
        compiled for 6.99.99.902, module version = 1.2.1
        Module class: X.Org Video Driver
        ABI class: X.Org Video Driver, version 0.8