Version:            (using Devel)
OS:                Linux
Installed from:    Compiled sources

I just installed the new KDE4.2rc1 packages for Kubuntu 8.10 and now I experience a kwin crash whenever I disable my primary display.
Having a laptop with 1024x768 resolution and an external monitor with 1280x1024, I usually manually switch disable the laptop display after login and set the resolution of my monitor to 1280x1024 (starts with 1024x768).
The resolution change on my external monitor works fine, but disabling laptop display, either via xrandr or via System Settings/Display crashes kwin with the following backtrace:

Application: KWin (kwin), signal SIGABRT
0x00007fd590e125f0 in __nanosleep_nocancel () from /lib/libc.so.6

Thread 1 (Thread 0x7fd596261700 (LWP 13956)):
[KCrash Handler]
#5  0x00007fd590d9cfd5 in raise () from /lib/libc.so.6
#6  0x00007fd590d9eb43 in abort () from /lib/libc.so.6
#7  0x00007fd5938e26b5 in qt_message_output () from /usr/lib/libQtCore.so.4
#8  0x00007fd5938e27fd in qFatal () from /usr/lib/libQtCore.so.4
#9  0x00007fd595e245d7 in KWin::Workspace::clientArea (this=0x1168800, opt=KWin::ScreenArea, screen=1, desktop=1) at /usr/include/qt4/QtCore/qvector.h:321
#10 0x00007fd595e247e2 in KWin::Workspace::clientArea (this=0x1168800, opt=KWin::ScreenArea, p=<value optimized out>, desktop=-1) at /build/buildd/kdebase-workspace-4.1.96/kwin/geometry.cpp:279
#11 0x00007fd595e24bf1 in KWin::Workspace::clientArea (this=0x1168800, opt=KWin::ScreenArea, c=0x1225a20) at /build/buildd/kdebase-workspace-4.1.96/kwin/geometry.cpp:284
#12 0x00007fd595e24ed0 in KWin::Client::adjustedClientArea (this=0x1225a20, desktopArea=@0x7fff9e297190, area=@0x7fff9e297190) at /build/buildd/kdebase-workspace-4.1.96/kwin/geometry.cpp:866
#13 0x00007fd595e2dd4b in KWin::Workspace::updateClientArea (this=0x1168800, force=false) at /build/buildd/kdebase-workspace-4.1.96/kwin/geometry.cpp:115
#14 0x00007fd595e2f06b in KWin::Workspace::desktopResized (this=0x1168800) at /build/buildd/kdebase-workspace-4.1.96/kwin/geometry.cpp:64
#15 0x00007fd595ddea65 in KWin::Workspace::qt_metacall (this=0x1168800, _c=QMetaObject::InvokeMetaMethod, _id=<value optimized out>, _a=0x7fff9e2973a0)
    at /build/buildd/kdebase-workspace-4.1.96/obj-x86_64-linux-gnu/kwin/workspace.moc:431
#16 0x00007fd5939e7134 in QMetaObject::activate () from /usr/lib/libQtCore.so.4
#17 0x00007fd594a9be6f in Kephal::Screens::screenAdded (this=0x3684, _t1=0x11f7d20) at /build/buildd/kdebase-workspace-4.1.96/obj-x86_64-linux-gnu/libs/kephal/libkephal/moc_screens.cxx:128
#18 0x00007fd594a8b607 in Kephal::DBusScreens::screenAddedSlot (this=0x11341a0, id=1) at /build/buildd/kdebase-workspace-4.1.96/libs/kephal/screens/dbus/dbusscreens.cpp:115
#19 0x00007fd594a9c2bf in Kephal::DBusScreens::qt_metacall (this=0x11341a0, _c=QMetaObject::InvokeMetaMethod, _id=2, _a=0x7fff9e297720)
    at /build/buildd/kdebase-workspace-4.1.96/obj-x86_64-linux-gnu/libs/kephal/libkephal/moc_dbusscreens.cxx:72
#20 0x00007fd5939e7134 in QMetaObject::activate () from /usr/lib/libQtCore.so.4
#21 0x00007fd594a9dc5e in OrgKdeKephalScreensInterface::screenAdded (this=0x3684, _t1=1) at /build/buildd/kdebase-workspace-4.1.96/obj-x86_64-linux-gnu/libs/kephal/libkephal/screens_interface.moc:109
#22 0x00007fd594a9e4ce in OrgKdeKephalScreensInterface::qt_metacall (this=0x11202e0, _c=QMetaObject::InvokeMetaMethod, _id=<value optimized out>, _a=0x7fff9e297cc0)
    at /build/buildd/kdebase-workspace-4.1.96/obj-x86_64-linux-gnu/libs/kephal/libkephal/screens_interface.moc:83
#23 0x00007fd593ce0ea3 in ?? () from /usr/lib/libQtDBus.so.4
#24 0x00007fd593ce7eef in ?? () from /usr/lib/libQtDBus.so.4
#25 0x00007fd5939e1da5 in QObject::event () from /usr/lib/libQtCore.so.4
#26 0x00007fd5940f6c3d in QApplicationPrivate::notify_helper () from /usr/lib/libQtGui.so.4
#27 0x00007fd5940fe9ba in QApplication::notify () from /usr/lib/libQtGui.so.4
#28 0x00007fd59557c3cb in KApplication::notify (this=0x7fff9e298710, receiver=0x11202e0, event=0x14ad2f0) at /build/buildd/kde4libs-4.1.96/kdeui/kernel/kapplication.cpp:307
#29 0x00007fd5939d2d61 in QCoreApplication::notifyInternal () from /usr/lib/libQtCore.so.4
#30 0x00007fd5939d39fa in QCoreApplicationPrivate::sendPostedEvents () from /usr/lib/libQtCore.so.4
#31 0x00007fd5939fb4d3 in ?? () from /usr/lib/libQtCore.so.4
#32 0x00007fd58f702d3b in g_main_context_dispatch () from /usr/lib/libglib-2.0.so.0
#33 0x00007fd58f70650d in ?? () from /usr/lib/libglib-2.0.so.0
#34 0x00007fd58f7066cb in g_main_context_iteration () from /usr/lib/libglib-2.0.so.0
#35 0x00007fd5939fb15f in QEventDispatcherGlib::processEvents () from /usr/lib/libQtCore.so.4
#36 0x00007fd594188a6f in ?? () from /usr/lib/libQtGui.so.4
#37 0x00007fd5939d1682 in QEventLoop::processEvents () from /usr/lib/libQtCore.so.4
#38 0x00007fd5939d180d in QEventLoop::exec () from /usr/lib/libQtCore.so.4
#39 0x00007fd5939d3cbd in QCoreApplication::exec () from /usr/lib/libQtCore.so.4
#40 0x00007fd595df814f in kdemain (argc=3, argv=0x7fff9e298be8) at /build/buildd/kdebase-workspace-4.1.96/kwin/main.cpp:519
#41 0x00007fd590d88466 in __libc_start_main () from /lib/libc.so.6
#42 0x00000000004007e9 in _start ()

4.2beta1 has worked fine for me.