The following issue occurs on my Kubuntu 11.10 system (Lenovo T420s w/ Intel HD Graphics 3000).

With KWin desktop effects enabled, Xorg crashes on each second login.

$ uname -a
Linux orion 3.1.0-0301rc10-generic #201110181253 SMP Tue Oct 18 12:57:33 UTC 2011 x86_64 x86_64 x86_64 GNU/Linux

$ lspci -vv
00:02.0 VGA compatible controller: Intel Corporation 2nd Generation Core Processor Family Integrated Graphics Controller (rev 09) (prog-if 00 [VGA controller])
        Subsystem: Lenovo Device 21d2
        Control: I/O+ Mem+ BusMaster+ SpecCycle- MemWINV- VGASnoop- ParErr- Stepping- SERR- FastB2B- DisINTx+
        Status: Cap+ 66MHz- UDF- FastB2B+ ParErr- DEVSEL=fast >TAbort- <TAbort- <MAbort- >SERR- <PERR- INTx-
        Latency: 0
        Interrupt: pin A routed to IRQ 49
        Region 0: Memory at f0000000 (64-bit, non-prefetchable) [size=4M]
        Region 2: Memory at e0000000 (64-bit, prefetchable) [size=256M]
        Region 4: I/O ports at 4000 [size=64]
        Expansion ROM at <unassigned> [disabled]
        Capabilities: <access denied>
        Kernel driver in use: i915
        Kernel modules: i915

Full backtrace (based on commit 2608a367acba7247e50754c3daeed09ba2e97d05):
-- 8< --
#0  0x00007fcbf4fa63a5 in __GI_raise (sig=6) at ../nptl/sysdeps/unix/sysv/linux/raise.c:64
        resultvar = 0
        pid = <optimized out>
        selftid = <optimized out>
#1  0x00007fcbf4fa9b0b in __GI_abort () at abort.c:92
        save_stage = 2
        act = {__sigaction_handler = {sa_handler = 0x4, sa_sigaction = 0x4}, sa_mask = {__val = {5, 
              140735046524302, 10, 140513966347903, 3, 140735046516714, 6, 140513966347907, 2, 
              140735046516734, 2, 140513966338901, 1, 140513966347903, 3, 140735046516708}}, sa_flags = 12, 
          sa_restorer = 0x7fcbf50cd683}
        sigs = {__val = {32, 0 <repeats 15 times>}}
#2  0x00007fcbf4fde113 in __libc_message (do_abort=2, 
    fmt=0x7fcbf50cf0d8 "*** glibc detected *** %s: %s: 0x%s ***\n")
    at ../sysdeps/unix/sysv/linux/libc_fatal.c:189
        ap = {{gp_offset = 40, fp_offset = 48, overflow_arg_area = 0x7fff6e748960, 
            reg_save_area = 0x7fff6e748870}}
        ap_copy = {{gp_offset = 16, fp_offset = 48, overflow_arg_area = 0x7fff6e748960, 
            reg_save_area = 0x7fff6e748870}}
        fd = 2
        on_2 = <optimized out>
        list = <optimized out>
        nlist = <optimized out>
        cp = <optimized out>
        written = <optimized out>
#3  0x00007fcbf4fe8a96 in malloc_printerr (action=3, str=0x7fcbf50cf278 "double free or corruption (!prev)", 
    ptr=<optimized out>) at malloc.c:6283
        buf = "0000000002bf2230"
        cp = <optimized out>
#4  0x00007fcbf4fecd7c in __GI___libc_free (mem=<optimized out>) at malloc.c:3738
        ar_ptr = 0x7fcbf530a1c0
        p = <optimized out>
        hook = <optimized out>
#5  0x00007fcbf31ae5de in i830_dri2_frame_event_drawable_gone (data=0x2bf2230, id=29360131)
    at ../../src/intel_dri.c:678
        resource = 0x2bf2230
#6  0x000000000044ea5c in FreeClientResources (client=0x2d01c10) at ../../dix/resource.c:854
        rtype = <optimized out>
        resources = <optimized out>
        this = 0x2f4b0c0
        j = <optimized out>
#7  0x000000000042f05a in CloseDownClient (client=0x2d01c10) at ../../dix/dispatch.c:3461
        really_close_down = <optimized out>
#8  0x000000000042fb9e in Dispatch () at ../../dix/dispatch.c:441
        clientReady = 0x29f4830
        result = <optimized out>
        client = 0x2d01c10
        nready = 0
        icheck = 0x7f1470
        start_tick = 680
#9  0x00000000004232fe in main (argc=8, argv=<optimized out>, envp=<optimized out>) at ../../dix/main.c:287
        i = <optimized out>
        alwaysCheckForInput = {0, 1}
-- 8< --

I bi-sected xserver-xorg-video-intel (as available at git://git.debian.org/git/pkg-xorg/driver/xserver-xorg-video-intel, which is what the Ubuntu packages are based on). The first bad commit is:

commit 2608a367acba7247e50754c3daeed09ba2e97d05
Author: Chris Wilson <chris@chris-wilson.co.uk>
Date:   Mon Jul 11 16:28:15 2011 +0100

    dri: Prevent abuse of the Resource database
    
    The Resource database is only designed to store a single value for a
    particular type associated with an XID. Due to the asynchronous nature
    of the vblank/flip requests, we would often associate multiple frame
    events with a particular drawable/client. Upon freeing the resource, we
    would not necessarily decouple the right value, leaving a stale pointer
    behind. Later when the client disappeared, we would write through that
    stale pointer upsetting valgrind and causing memory corruption. MDK.
    
    Instead, we need to implement an extra layer for tracking multiple
    frames within a single Resource.
    
    Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=37700
    Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>