Application that crashed: kwin
Version of the application: 4.2.96 (KDE 4.2.96 (KDE 4.3 RC2))
KDE Version: 4.2.96 (KDE 4.2.96 (KDE 4.3 RC2))
Qt Version: 4.5.0
Operating System: Linux 2.6.28-13-generic x86_64
Distribution: Ubuntu 9.04

What I was doing when the application crashed:
Perhaps this is a duplicate of other bugs, I'm not sure it's reproducible.  I was closing firefox and, probably, alt-tabbed (cover switch effect) to another window.  The display locked up, window borders disappeared, a "firefox is not responding" window popped up, and eventually kwin restarted and firefox closed.

 -- Backtrace:
Application: KWin (kwin), signal: Segmentation fault
[KCrash Handler]
#5  WindowPaintData (this=0x7fff2a8ed690, w=0x1) at /build/buildd/kdebase-workspace-4.2.96/kwin/lib/kwineffects.cpp:76
#6  0x00007f3b0fa8c698 in KWin::CoverSwitchEffect::paintFrontWindow (this=0xe41ab0, frontWindow=0x1, width=1920, leftWindows=3, rightWindows=3, reflectedWindow=false)
    at /build/buildd/kdebase-workspace-4.2.96/kwin/effects/coverswitch/coverswitch.cpp:792
#7  0x00007f3b0fa8d817 in KWin::CoverSwitchEffect::paintScene (this=0xe41ab0, frontWindow=0x1, leftWindows=0xe41b30, rightWindows=0xe41b38, reflectedWindows=<value optimized out>)
    at /build/buildd/kdebase-workspace-4.2.96/kwin/effects/coverswitch/coverswitch.cpp:455
#8  0x00007f3b0fa93a0b in KWin::CoverSwitchEffect::paintScreen (this=0xe41ab0, mask=0, region=
        {d = 0x7fff2a8ed9f0, static shared_empty = {ref = {_q_value = 15933}, rgn = 0x0, xrectangles = 0x0, qt_rgn = 0x0}}, data=<value optimized out>)
    at /build/buildd/kdebase-workspace-4.2.96/kwin/effects/coverswitch/coverswitch.cpp:308
#9  0x00007f3b224a5247 in KWin::EffectsHandlerImpl::paintScreen (this=0xd01310, mask=32, region=
        {d = 0x7fff2a8eda40, static shared_empty = {ref = {_q_value = 15933}, rgn = 0x0, xrectangles = 0x0, qt_rgn = 0x0}}, data=@0x7fff2a8edce0)
    at /build/buildd/kdebase-workspace-4.2.96/kwin/effects.cpp:122
#10 0x00007f3b21fc7660 in KWin::Effect::paintScreen (this=<value optimized out>, mask=32, region=<value optimized out>, data=@0x7fff2a8edce0)
    at /build/buildd/kdebase-workspace-4.2.96/kwin/lib/kwineffects.cpp:206
#11 0x00007f3b224a5247 in KWin::EffectsHandlerImpl::paintScreen (this=0xd01310, mask=32, region=
        {d = 0x7fff2a8edb30, static shared_empty = {ref = {_q_value = 15933}, rgn = 0x0, xrectangles = 0x0, qt_rgn = 0x0}}, data=@0x7fff2a8edce0)
    at /build/buildd/kdebase-workspace-4.2.96/kwin/effects.cpp:122
#12 0x00007f3b0fab4ab5 in KWin::TrackMouseEffect::paintScreen (this=0xe52dd0, mask=32, region=
        {d = 0x7fff2a8edba0, static shared_empty = {ref = {_q_value = 15933}, rgn = 0x0, xrectangles = 0x0, qt_rgn = 0x0}}, data=@0x7fff2a8edce0)
    at /build/buildd/kdebase-workspace-4.2.96/kwin/effects/trackmouse/trackmouse.cpp:71
#13 0x00007f3b224a5247 in KWin::EffectsHandlerImpl::paintScreen (this=0xd01310, mask=32, region=
        {d = 0x7fff2a8edc00, static shared_empty = {ref = {_q_value = 15933}, rgn = 0x0, xrectangles = 0x0, qt_rgn = 0x0}}, data=@0x7fff2a8edce0)
    at /build/buildd/kdebase-workspace-4.2.96/kwin/effects.cpp:122
#14 0x00007f3b0fa8acb5 in KWin::ZoomEffect::paintScreen (this=<value optimized out>, mask=32, region=
        {d = 0x7fff2a8edc70, static shared_empty = {ref = {_q_value = 15933}, rgn = 0x0, xrectangles = 0x0, qt_rgn = 0x0}}, data=@0x7fff2a8edce0)
    at /build/buildd/kdebase-workspace-4.2.96/kwin/effects/zoom/zoom.cpp:73
#15 0x00007f3b224a5247 in KWin::EffectsHandlerImpl::paintScreen (this=0xd01310, mask=32, region=
        {d = 0x7fff2a8edd60, static shared_empty = {ref = {_q_value = 15933}, rgn = 0x0, xrectangles = 0x0, qt_rgn = 0x0}}, data=@0x7fff2a8edce0)
    at /build/buildd/kdebase-workspace-4.2.96/kwin/effects.cpp:122
#16 0x00007f3b22490f4c in KWin::Scene::paintScreen (this=0xd51cf0, mask=0x7fff2a8ede6c, region=0x7fff2a8edf70) at /build/buildd/kdebase-workspace-4.2.96/kwin/scene.cpp:134
#17 0x00007f3b2249f93f in KWin::SceneOpenGL::paint (this=0xd51cf0, damage={d = 0x7fff2a8edf70, static shared_empty = {ref = {_q_value = 15933}, rgn = 0x0, xrectangles = 0x0, qt_rgn = 0x0}}, 
    toplevels=<value optimized out>) at /build/buildd/kdebase-workspace-4.2.96/kwin/scene_opengl.cpp:746
#18 0x00007f3b2248c2f6 in KWin::Workspace::performCompositing (this=0xd7cd00) at /build/buildd/kdebase-workspace-4.2.96/kwin/composite.cpp:420
#19 0x00007f3b22427e8a in KWin::Workspace::qt_metacall (this=0xd7cd00, _c=QMetaObject::InvokeMetaMethod, _id=<value optimized out>, _a=0x7fff2a8ee100)
    at /build/buildd/kdebase-workspace-4.2.96/obj-x86_64-linux-gnu/kwin/workspace.moc:473
#20 0x00007f3b1fb001f2 in QMetaObject::activate (sender=0xd7d128, from_signal_index=<value optimized out>, to_signal_index=4, argv=0x3) at kernel/qobject.cpp:3069
#21 0x00007f3b1fafa4d3 in QObject::event (this=0xd7d128, e=0x1) at kernel/qobject.cpp:1082
#22 0x00007f3b2022f78d in QApplicationPrivate::notify_helper (this=0xaed970, receiver=0xd7d128, e=0x7fff2a8ee7b0) at kernel/qapplication.cpp:4084
#23 0x00007f3b2023797a in QApplication::notify (this=0x7fff2a8eeb20, receiver=0xd7d128, e=0x7fff2a8ee7b0) at kernel/qapplication.cpp:4049
#24 0x00007f3b2179b63b in KApplication::notify (this=0x7fff2a8eeb20, receiver=0xd7d128, event=0x7fff2a8ee7b0) at /build/buildd/kde4libs-4.2.96/kdeui/kernel/kapplication.cpp:302
#25 0x00007f3b1faea75c in QCoreApplication::notifyInternal (this=0x7fff2a8eeb20, receiver=0xd7d128, event=0x7fff2a8ee7b0) at kernel/qcoreapplication.cpp:602
#26 0x00007f3b1fb177f6 in QTimerInfoList::activateTimers (this=0xaf1560) at ../../include/QtCore/../../src/corelib/kernel/qcoreapplication.h:213
#27 0x00007f3b1fb13f0d in timerSourceDispatch (source=<value optimized out>) at kernel/qeventdispatcher_glib.cpp:164
#28 0x00007f3b18dff20a in g_main_context_dispatch () from /usr/lib/libglib-2.0.so.0
#29 0x00007f3b18e028e0 in ?? () from /usr/lib/libglib-2.0.so.0
#30 0x00007f3b18e02a7c in g_main_context_iteration () from /usr/lib/libglib-2.0.so.0
#31 0x00007f3b1fb13e6f in QEventDispatcherGlib::processEvents (this=0xaedb80, flags=<value optimized out>) at kernel/qeventdispatcher_glib.cpp:323
#32 0x00007f3b202c7bef in QGuiEventDispatcherGlib::processEvents (this=0x1, flags=<value optimized out>) at kernel/qguieventdispatcher_glib.cpp:202
#33 0x00007f3b1fae9002 in QEventLoop::processEvents (this=<value optimized out>, flags={i = 714009120}) at kernel/qeventloop.cpp:149
#34 0x00007f3b1fae93cd in QEventLoop::exec (this=0x7fff2a8eea60, flags={i = 714009200}) at kernel/qeventloop.cpp:200
#35 0x00007f3b1faeb694 in QCoreApplication::exec () at kernel/qcoreapplication.cpp:880
#36 0x00007f3b22440048 in kdemain (argc=3, argv=0x7fff2a8ef008) at /build/buildd/kdebase-workspace-4.2.96/kwin/main.cpp:527
#37 0x00007f3b1cf435a6 in __libc_start_main (main=0x4008c0 <main>, argc=3, ubp_av=0x7fff2a8ef008, init=0x4008f0 <__libc_csu_init>, fini=<value optimized out>, rtld_fini=<value optimized out>, 
    stack_end=0x7fff2a8eeff8) at libc-start.c:220
#38 0x00000000004007f9 in _start () at ../sysdeps/x86_64/elf/start.S:113

Reported using DrKonqi