Application that crashed: kwin
Version of the application: 4.3.00 (KDE 4.3.0)
KDE Version: 4.3.00 (KDE 4.3.0)
Qt Version: 4.5.2
Operating System: Linux 2.6.29-2-686 i686
Distribution: Debian GNU/Linux unstable (sid)

What I was doing when the application crashed:
with a few applications open (icedove and iceweasel, debian's unbranded thunderbird and firefox, and skype), no flash running, almost zero load on the system, ALT+TAB switching with the "cover switch" effect enabled resulted in kwin's death. 

 -- Backtrace:
Application: KWin (kwin), signal: Segmentation fault
[KCrash Handler]
#6  WindowPaintData (this=0xbf9faac8, w=0x400001) at ../../../kwin/lib/kwineffects.cpp:76
#7  0xb1eaaa17 in KWin::CoverSwitchEffect::paintWindows (this=0x866afa8, windows=0x866b008, left=true, reflectedWindows=false, additionalWindow=0x0)
    at ../../../kwin/effects/coverswitch/coverswitch.cpp:886
#8  0xb1eab259 in KWin::CoverSwitchEffect::paintScene (this=0x866afa8, frontWindow=0x84715d8, leftWindows=0x866b008, rightWindows=0x866b00c, reflectedWindows=false)
    at ../../../kwin/effects/coverswitch/coverswitch.cpp:427
#9  0xb1eb1acd in KWin::CoverSwitchEffect::paintScreen (this=0x866afa8, mask=32, region=..., data=...) at ../../../kwin/effects/coverswitch/coverswitch.cpp:308
#10 0xb80a8f4f in KWin::EffectsHandlerImpl::paintScreen (this=0x85be588, mask=32, region=..., data=...) at ../../kwin/effects.cpp:122
#11 0xb78d3596 in KWin::Effect::paintScreen (this=0x8667370, mask=32, region=..., data=...) at ../../../kwin/lib/kwineffects.cpp:206
#12 0xb80a8f4f in KWin::EffectsHandlerImpl::paintScreen (this=0x85be588, mask=32, region=..., data=...) at ../../kwin/effects.cpp:122
#13 0xb78d3596 in KWin::Effect::paintScreen (this=0x8657978, mask=32, region=..., data=...) at ../../../kwin/lib/kwineffects.cpp:206
#14 0xb80a8f4f in KWin::EffectsHandlerImpl::paintScreen (this=0x85be588, mask=32, region=..., data=...) at ../../kwin/effects.cpp:122
#15 0xb1ea876d in KWin::ZoomEffect::paintScreen (this=0x87406e8, mask=32, region=..., data=...) at ../../../kwin/effects/zoom/zoom.cpp:73
#16 0xb80a8f4f in KWin::EffectsHandlerImpl::paintScreen (this=0x85be588, mask=32, region=..., data=...) at ../../kwin/effects.cpp:122
#17 0xb1ea5f5f in KWin::ThumbnailAsideEffect::paintScreen (this=0x867cfe0, mask=32, region=..., data=...) at ../../../kwin/effects/thumbnailaside/thumbnailaside.cpp:56
#18 0xb80a8f4f in KWin::EffectsHandlerImpl::paintScreen (this=0x85be588, mask=32, region=..., data=...) at ../../kwin/effects.cpp:122
#19 0xb8092d20 in KWin::Scene::paintScreen (this=0x858b198, mask=0xbf9fb234, region=0xbf9fb2f4) at ../../kwin/scene.cpp:134
#20 0xb80a2697 in KWin::SceneOpenGL::paint (this=0x858b198, damage=..., toplevels=...) at ../../kwin/scene_opengl.cpp:746
#21 0xb808dd26 in KWin::Workspace::performCompositing (this=0x8547fa8) at ../../kwin/composite.cpp:420
#22 0xb801fa8e in KWin::Workspace::qt_metacall (this=0x8547fa8, _c=QMetaObject::InvokeMetaMethod, _id=149, _a=0xbf9fb3c8) at ./workspace.moc:473
#23 0xb6987ad3 in QMetaObject::activate (sender=0x854820c, from_signal_index=4, to_signal_index=4, argv=0x0) at kernel/qobject.cpp:3112
#24 0xb6988722 in QMetaObject::activate (sender=0x854820c, m=0xb6a63d84, local_signal_index=0, argv=0x0) at kernel/qobject.cpp:3186
#25 0xb69c3117 in QTimer::timeout (this=0x854820c) at .moc/release-shared/moc_qtimer.cpp:128
#26 0xb698de3e in QTimer::timerEvent (this=0x854820c, e=0xbf9fb86c) at kernel/qtimer.cpp:261
#27 0xb6982b6f in QObject::event (this=0x854820c, e=0xbf9fb86c) at kernel/qobject.cpp:1074
#28 0xb6c1e7d4 in QApplicationPrivate::notify_helper (this=0x841ad08, receiver=0x854820c, e=0xbf9fb86c) at kernel/qapplication.cpp:4056
#29 0xb6c2693e in QApplication::notify (this=0xbf9fbb44, receiver=0x854820c, e=0xbf9fb86c) at kernel/qapplication.cpp:3603
#30 0xb7ce700d in KApplication::notify (this=0xbf9fbb44, receiver=0x854820c, event=0xbf9fb86c) at ../../kdeui/kernel/kapplication.cpp:302
#31 0xb8036ffd in KWin::Application::notify (this=0xbf9fbb44, o=0x854820c, e=0xbf9fb86c) at ../../kwin/main.cpp:371
#32 0xb697296b in QCoreApplication::notifyInternal (this=0xbf9fbb44, receiver=0x854820c, event=0xbf9fb86c) at kernel/qcoreapplication.cpp:610
#33 0xb69a1301 in QCoreApplication::sendEvent (this=0x841da8c) at ../../include/QtCore/../../src/corelib/kernel/qcoreapplication.h:213
#34 QTimerInfoList::activateTimers (this=0x841da8c) at kernel/qeventdispatcher_unix.cpp:572
#35 0xb699d8a0 in timerSourceDispatch (source=0x841da58) at kernel/qeventdispatcher_glib.cpp:165
#36 0xb484d368 in g_main_context_dispatch () from /usr/lib/libglib-2.0.so.0
#37 0xb48508c3 in ?? () from /usr/lib/libglib-2.0.so.0
#38 0xb4850a48 in g_main_context_iteration () from /usr/lib/libglib-2.0.so.0
#39 0xb699d7f8 in QEventDispatcherGlib::processEvents (this=0x84045b0, flags=...) at kernel/qeventdispatcher_glib.cpp:327
#40 0xb6cbdf85 in QGuiEventDispatcherGlib::processEvents (this=0x84045b0, flags=...) at kernel/qguieventdispatcher_glib.cpp:202
#41 0xb6970fba in QEventLoop::processEvents (this=0xbf9fba90, flags=...) at kernel/qeventloop.cpp:149
#42 0xb6971402 in QEventLoop::exec (this=0xbf9fba90, flags=...) at kernel/qeventloop.cpp:201
#43 0xb6973859 in QCoreApplication::exec () at kernel/qcoreapplication.cpp:888
#44 0xb6c1e657 in QApplication::exec () at kernel/qapplication.cpp:3525
#45 0xb803968f in kdemain (argc=3, argv=0xbf9fbd24) at ../../kwin/main.cpp:527
#46 0x080485b2 in main (argc=10157096, argv=0xb76e2ae8) at kwin_dummy.cpp:3

This bug may be a duplicate of or related to bug 202644

Reported using DrKonqi