Comment 18 for bug 351756

A backtrace is available here:
http://launchpadlibrarian.net/24551278/ThreadStacktrace.txt

briefly:

#0 0xb78e3309 in drm_intel_bo_reference () from /usr/lib/libdrm_intel.so.1
#1 0xb794588b in i830_set_pixmap_bo (pixmap=0x8465190, bo=0x0)
    at ../../src/i830_exa.c:772
 old_bo = <value optimized out>
#2 0xb7920ab8 in i830_xf86crtc_resize (scrn=0x83efef0, width=1152,
    height=864) at ../../src/i830_driver.c:1178
 new_front = <value optimized out>
 old_front = (i830_memory *) 0x843e810
 mem_box = {x1 = 0, y1 = 0, x2 = 2048, y2 = 864}
 screen = (ScreenPtr) 0x843e350
 old_x = 3120
 old_y = 1050
 old_width = 4096