(I've no idea if I filled the bug details correctly.  I just guessed.)

I reported this bug in bugs.kde.org, but I've been told it's a bug in X and to report it here.

Application: kwin (4.4.92 (KDE 4.4.92 (KDE 4.5 RC2)))
KDE Platform Version: 4.4.92 (KDE 4.4.92 (KDE 4.5 RC2)) (Compiled from sources)
Qt Version: 4.7.0
Operating System: Linux 2.6.35-rc5 x86_64
Distribution: "Gentoo Base System release 2.0.1"

Kernel 2.6.35-rc5
xorg-server Git master
xf86-video-ati Git master, using DRI2/KMS
Mesa Git master
libdrm from Git master

-- Information about the crash:
In System Settings, when I change some options in the "Advanced" tab of
"Desktop Effects" (like changing "VSync" and "direct rendering"), KWin crashes.

The crash can be reproduced every time.

-- Backtrace:
Application: KWin (kwin), signal: Segmentation fault
[KCrash Handler]
#6  DRI2WireToEvent (dpy=<value optimized out>, event=<value optimized out>,
wire=<value optimized out>) at dri2.c:105
#7  0x0000003002444f1e in _XEnq (dpy=<value optimized out>, event=<value
optimized out>) at XlibInt.c:2431
#8  0x000000300244ca13 in handle_response (dpy=<value optimized out>,
response=<value optimized out>, in_XReply=<value optimized out>) at
xcb_io.c:279
#9  0x000000300244cfa0 in _XReply (dpy=<value optimized out>, rep=<value
optimized out>, extra=<value optimized out>, discard=<value optimized out>) at
xcb_io.c:554
#10 0x00000030024409e2 in XSync (dpy=<value optimized out>, discard=<value
optimized out>) at Sync.c:46
#11 0x0000003008c5296f in DRI2DestroyDrawable (dpy=<value optimized out>,
drawable=<value optimized out>) at dri2.c:350
#12 0x0000003008c51584 in dri2DestroyDrawable (pdraw=<value optimized out>) at
dri2_glx.c:174
#13 0x0000003008c4bab3 in DestroyDRIDrawable (dpy=<value optimized out>,
drawable=<value optimized out>, destroy_xdrawable=<value optimized out>) at
glx_pbuffer.c:225
#14 0x0000003080ac4dc2 in KWin::SceneOpenGL::Texture::release (this=<value
optimized out>) at
/var/tmp/portage/kde-base/kwin-4.4.92/work/kwin-4.4.92/kwin/scene_opengl.cpp:1009
#15 0x0000003080ac5151 in KWin::SceneOpenGL::Texture::discard (this=<value
optimized out>) at
/var/tmp/portage/kde-base/kwin-4.4.92/work/kwin-4.4.92/kwin/scene_opengl.cpp:999
#16 0x0000003080ac516d in KWin::SceneOpenGL::Window::discardTexture
(this=<value optimized out>) at
/var/tmp/portage/kde-base/kwin-4.4.92/work/kwin-4.4.92/kwin/scene_opengl.cpp:1331
#17 0x0000003080ac5283 in ~Window (this=<value optimized out>, __in_chrg=<value
optimized out>) at
/var/tmp/portage/kde-base/kwin-4.4.92/work/kwin-4.4.92/kwin/scene_opengl.cpp:1304
#18 0x0000003080ac67dc in KWin::SceneOpenGL::windowClosed (this=<value
optimized out>, c=<value optimized out>, deleted=<value optimized out>)
    at
/var/tmp/portage/kde-base/kwin-4.4.92/work/kwin-4.4.92/kwin/scene_opengl.cpp:933
#19 0x0000003080ab30d9 in KWin::Workspace::finishCompositing (this=<value
optimized out>) at
/var/tmp/portage/kde-base/kwin-4.4.92/work/kwin-4.4.92/kwin/composite.cpp:258
#20 0x0000003080ab6489 in KWin::Workspace::fallbackToXRenderCompositing
(this=<value optimized out>) at
/var/tmp/portage/kde-base/kwin-4.4.92/work/kwin-4.4.92/kwin/composite.cpp:301
#21 0x0000003080a36768 in KWin::Workspace::qt_metacall (this=<value optimized
out>, _c=<value optimized out>, _id=<value optimized out>, _a=<value optimized
out>)
    at
/var/tmp/portage/kde-base/kwin-4.4.92/work/kwin-4.4.92_build/kwin/workspace.moc:582
#22 0x0000003009f77bbe in QObject::event (this=<value optimized out>, e=<value
optimized out>) at kernel/qobject.cpp:1211
#23 0x00000030137b031c in QApplicationPrivate::notify_helper (this=<value
optimized out>, receiver=<value optimized out>, e=<value optimized out>) at
kernel/qapplication.cpp:4389
#24 0x00000030137b5e8d in QApplication::notify (this=<value optimized out>,
receiver=<value optimized out>, e=<value optimized out>) at
kernel/qapplication.cpp:4270
#25 0x0000003015639c36 in KApplication::notify (this=<value optimized out>,
receiver=<value optimized out>, event=<value optimized out>)
    at
/var/tmp/portage/kde-base/kdelibs-4.4.92/work/kdelibs-4.4.92/kdeui/kernel/kapplication.cpp:309
#26 0x0000003009f65f3b in QCoreApplication::notifyInternal (this=<value
optimized out>, receiver=<value optimized out>, event=<value optimized out>) at
kernel/qcoreapplication.cpp:732
#27 0x0000003009f68e41 in QCoreApplication::sendEvent (receiver=<value
optimized out>, event_type=<value optimized out>, data=<value optimized out>)
at kernel/qcoreapplication.h:215
#28 QCoreApplicationPrivate::sendPostedEvents (receiver=<value optimized out>,
event_type=<value optimized out>, data=<value optimized out>) at
kernel/qcoreapplication.cpp:1373
#29 0x0000003013861214 in QCoreApplication::sendPostedEvents (this=<value
optimized out>, flags=<value optimized out>) at
../../include/QtCore/../../src/corelib/kernel/qcoreapplication.h:220
#30 QEventDispatcherX11::processEvents (this=<value optimized out>,
flags=<value optimized out>) at kernel/qeventdispatcher_x11.cpp:75
#31 0x0000003009f64c92 in QEventLoop::processEvents (this=<value optimized
out>, flags=<value optimized out>) at kernel/qeventloop.cpp:149
#32 0x0000003009f65074 in QEventLoop::exec (this=<value optimized out>,
flags=<value optimized out>) at kernel/qeventloop.cpp:201
#33 0x0000003009f690cb in QCoreApplication::exec () at
kernel/qcoreapplication.cpp:1009
#34 0x0000003080a52d8d in kdemain (argc=<value optimized out>, argv=<value
optimized out>) at
/var/tmp/portage/kde-base/kwin-4.4.92/work/kwin-4.4.92/kwin/main.cpp:531
#35 0x000000300041ebbd in __libc_start_main (main=<value optimized out>,
argc=<value optimized out>, ubp_av=<value optimized out>, init=<value optimized
out>, fini=<value optimized out>, 
    rtld_fini=<value optimized out>, stack_end=) at libc-start.c:226
#36 0x00000000004006c9 in _start ()