I'm a Kubuntu user, and when I checked "Use VSync" setting in System Settings>Desktop Effects>Advanced, Kwin crashed. I filed a bug against Kwin (https://bugs.kde.org/show_bug.cgi?id=245238), but developers pointed out it was a driver bug, and encouraged me to file a bug here against the driver.

As pointed out during the exchange, there seems to be a general problem when re/creating a gl context on the driver, as also bug 244820 (https://bugs.kde.org/show_bug.cgi?id=244820) seems to confirm, which appears to have a different backtrace but similar context. As for the backtrace of my bug report, I'll paste it over here for ease of reference:

-- Backtrace:
Application: KWin (kwin), signal: Segmentation fault
[Current thread is 1 (Thread 0xb77d2720 (LWP 1462))]

Thread 2 (Thread 0xae681b70 (LWP 1482)):
#0  0x00bc7e46 in clock_gettime () from /lib/librt.so.1
#1  0x0474d7eb in do_gettime () at tools/qelapsedtimer_unix.cpp:105
#2  qt_gettime () at tools/qelapsedtimer_unix.cpp:119
#3  0x04823ac5 in QTimerInfoList::updateCurrentTime (this=0x8899028) at
kernel/qeventdispatcher_unix.cpp:339
#4  0x04824135 in QEventDispatcherUNIXPrivate::doSelect (this=0x8898988,
flags=..., timeout=0x0) at kernel/qeventdispatcher_unix.cpp:182
#5  0x04825005 in QEventDispatcherUNIX::processEvents (this=0x884d7c8,
flags=...) at kernel/qeventdispatcher_unix.cpp:919
#6  0x047f1cb9 in QEventLoop::processEvents (this=0xae681280, flags=) at
kernel/qeventloop.cpp:149
#7  0x047f213a in QEventLoop::exec (this=0xae681280, flags=...) at
kernel/qeventloop.cpp:201
#8  0x046efa8e in QThread::exec (this=0x885e078) at thread/qthread.cpp:490
#9  0x047d0eab in QInotifyFileSystemWatcherEngine::run (this=0x885e078) at
io/qfilesystemwatcher_inotify.cpp:248
#10 0x046f2d09 in QThreadPrivate::start (arg=0x885e078) at
thread/qthread_unix.cpp:266
#11 0x0085fcc9 in start_thread () from /lib/libpthread.so.0
#12 0x001e073e in clone () from /lib/libc.so.6

Thread 1 (Thread 0xb77d2720 (LWP 1462)):
[KCrash Handler]
#7  0x00811faa in dri2DrawableGetMSC (psc=0x81f0b00, pdraw=0x0, ust=0xbfcfbc78,
msc=0xbfcfbc70, sbc=0xbfcfbc68) at dri2_glx.c:212
#8  0x007ebcbc in __glXGetVideoSyncSGI (count=0xbfcfbcbc) at glxcmds.c:2182
#9  0x00b2d973 in KWin::SceneOpenGL::waitSync (this=0x8251c00) at
../../kwin/scene_opengl.cpp:781
#10 0x00b32387 in KWin::SceneOpenGL::flushBuffer (this=0x8251c00, mask=0,
damage=...) at ../../kwin/scene_opengl.cpp:830
#11 0x00b33434 in KWin::SceneOpenGL::paint (this=0x8251c00, damage=...,
toplevels=...) at ../../kwin/scene_opengl.cpp:761
#12 0x00b1b174 in KWin::Workspace::performCompositing (this=0x8264da0) at
../../kwin/composite.cpp:455
#13 0x00a8c2be in KWin::Workspace::qt_metacall (this=0x8264da0,
_c=QMetaObject::InvokeMetaMethod, _id=190, _a=0xbfcfbfdc) at
./workspace.moc:583
#14 0x047f8f3a in QMetaObject::metacall (object=0x8264da0, cl=8462208, idx=190,
argv=0xbfcfbfdc) at kernel/qmetaobject.cpp:237
#15 0x0480bccd in QMetaObject::activate (sender=0x8265050, m=0x49245a4,
local_signal_index=0, argv=0x0) at kernel/qobject.cpp:3280
#16 0x0485d1c7 in QTimer::timeout (this=0x8265050) at
.moc/release-shared/moc_qtimer.cpp:134
#17 0x048117de in QTimer::timerEvent (this=0x8265050, e=0xbfcfc530) at
kernel/qtimer.cpp:271
#18 0x04805b54 in QObject::event (this=0x8265050, e=0x81f0b00) at
kernel/qobject.cpp:1183
#19 0x0513208c in QApplicationPrivate::notify_helper (this=0x8184550,
receiver=0x8265050, e=0xbfcfc530) at kernel/qapplication.cpp:4389
#20 0x051380ce in QApplication::notify (this=0xbfcfc834, receiver=0x8265050,
e=0xbfcfc530) at kernel/qapplication.cpp:3791
#21 0x0045806a in KApplication::notify (this=0xbfcfc834, receiver=0x8265050,
event=0xbfcfc530) at ../../kdeui/kernel/kapplication.cpp:309
#22 0x00aa911f in KWin::Application::notify (this=0xbfcfc834, o=0x8265050,
e=0xbfcfc530) at ../../kwin/main.cpp:372
#23 0x047f31eb in QCoreApplication::notifyInternal (this=0xbfcfc834,
receiver=0x8265050, event=0xbfcfc530) at kernel/qcoreapplication.cpp:732
#24 0x04824eb6 in QCoreApplication::sendEvent (this=0x8184e60) at
../../include/QtCore/../../src/corelib/kernel/qcoreapplication.h:215
#25 QTimerInfoList::activateTimers (this=0x8184e60) at
kernel/qeventdispatcher_unix.cpp:602
#26 0x04825017 in QEventDispatcherUNIX::processEvents (this=0x8168af8,
flags=...) at kernel/qeventdispatcher_unix.cpp:923
#27 0x051f3f82 in QEventDispatcherX11::processEvents (this=0x8168af8,
flags=...) at kernel/qeventdispatcher_x11.cpp:152
#28 0x047f1cb9 in QEventLoop::processEvents (this=0xbfcfc784, flags=) at
kernel/qeventloop.cpp:149
#29 0x047f213a in QEventLoop::exec (this=0xbfcfc784, flags=...) at
kernel/qeventloop.cpp:201
#30 0x047f66bf in QCoreApplication::exec () at kernel/qcoreapplication.cpp:1009
#31 0x05130eb7 in QApplication::exec () at kernel/qapplication.cpp:3665
#32 0x00aab284 in kdemain (argc=1, argv=0xbfcfca94) at ../../kwin/main.cpp:531
#33 0x0804860b in main (argc=1, argv=0xbfcfca94) at kwin_dummy.cpp:3

Some additional details:

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))
Qt Version: 4.7.0
Operating System: Linux 2.6.35-9-generic i686
Distribution: Ubuntu maverick (development branch)
direct rendering: Yes
OpenGL renderer string: Mesa DRI Intel(R) 852GM/855GM GEM 20100328 2010Q1 x86/MMX/SSE2
server glx vendor string: SGI
client glx vendor string: Mesa Project and SGI
OpenGL vendor string: Tungsten Graphics, Inc

Please, let me know if I can be of further help.