I reported this bug first in the Debian BTS at http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=626646 .

Per the comments of Cyril Brulebois in that bug entry, I tested freedesktop.org xf86-video-intel git master merged to the Debian packaging repository of xserver-xorg-video-intel. It seems the bug is still present in master/HEAD of xf86-video-intel as of commit 9d6e02a1 (HEAD as of today May 28).

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I can pretty reproducibly either crash the X server or corrupt the display, sometimes permanently, by exiting the fullscreen mode of the proprietary flash player while it plays video. After compiling the X server with no optimizations and running it under valgrind, I believe the reason is a use after free in i830_dri2_frame_event_drawable_gone(). See http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=626646#10 .

Here is the valgrind error message which I get when exiting fullscreen mode (this message is against current git master, the one against the version in Debian unstable is in the Debian BTS message #10):

==29640== Invalid write of size 4
==29640==    at 0x8BCFD00: i830_dri2_frame_event_drawable_gone (intel_dri.c:598)
==29640==    by 0x45D38A: FreeResource (resource.c:596)
==29640==    by 0x42E38A: ProcDestroyWindow (dispatch.c:732)
==29640==    by 0x42D734: Dispatch (dispatch.c:431)
==29640==    by 0x425A3A: main (main.c:287)
==29640==  Address 0xcc80310 is 0 bytes inside a block of size 56 free'd
==29640==    at 0x40267AE: free (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==29640==    by 0x8BD0522: I830DRI2FrameEventHandler (intel_dri.c:844)
==29640==    by 0x8BAFDA9: intel_vblank_handler (intel_display.c:1517)
==29640==    by 0x8796A1A: drmHandleEvent (in /usr/lib/libdrm.so.2.4.0)
==29640==    by 0x8BAFEF6: drm_wakeup_handler (intel_display.c:1564)
==29640==    by 0x43B47D: WakeupHandler (dixutils.c:419)
==29640==    by 0x479090: WaitForSomething (WaitFor.c:235)
==29640==    by 0x42D55F: Dispatch (dispatch.c:367)
==29640==    by 0x425A3A: main (main.c:287)

Looking at the code, the offending write happens in intel_dri.c:

   593 static int
   594 i830_dri2_frame_event_drawable_gone(void *data, XID id)
   595 {
   596         DRI2FrameEventPtr       frame_event = data;
   597 
** 598         frame_event->drawable_id = None;
   599         return Success;
   600 }

And the frame_event structure presumably has been previously freed here in the same file:

   760 void I830DRI2FrameEventHandler(unsigned int frame, unsigned int tv_sec,
   761                                unsigned int tv_usec, DRI2FrameEventPtr swap_info)
   762 {
   763         DrawablePtr drawable;
   764         ScreenPtr screen;
   ...
   841         i830_dri2_del_frame_event(swap_info);
   842         I830DRI2DestroyBuffer(drawable, swap_info->front);
   843         I830DRI2DestroyBuffer(drawable, swap_info->back);
** 844         free(swap_info);
   845 }

GDB crash logs, glxinfo output, lspci output, Xorg.log, udev information, kernel log and lots of other details you probably do not care that much about are nonetheless available in my original bug report to the Debian BTS:
http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=626646#5

If you need any further information or debugging, I will be happy to be of assistance.