Version:            (using Devel)
Compiler:          gcc version 4.0.4 
OS:                Linux
Installed from:    Compiled sources

Was typing into a Konsole, and then hit the keystroke to switch to another desktop. Konsole got the keystroke, and then the kded4 crash handler came up. (In other words, this is not likely to be reproducible.)

Backtrace:

Application: KDE Daemon (kded4), signal SIGABRT
0x00007f3e329e9621 in nanosleep () from /lib/libc.so.6

Thread 1 (Thread 0x7f3e372b56f0 (LWP 6046)):
[KCrash Handler]
#5  0x00007f3e32973fd5 in raise () from /lib/libc.so.6
#6  0x00007f3e32975b43 in abort () from /lib/libc.so.6
#7  0x00007f3e35521bac in qt_message_output (msgType=QtFatalMsg, 
    buf=0x7fff3f2e7300 "ASSERT: \"dev->backendObject()==0\" in file /home/kdesvn/kdelibs/solid/solid/devicemanager.cpp, line 164")
    at /home/kdesvn/qt-copy/src/corelib/global/qglobal.cpp:2102
#8  0x00007f3e35521cbb in qFatal (msg=0x7f3e3566b160 "ASSERT: \"%s\" in file %s, line %d") at /home/kdesvn/qt-copy/src/corelib/global/qglobal.cpp:2303
#9  0x00007f3e35521d29 in qt_assert (assertion=0x7f3e31e44e8b "dev->backendObject()==0", file=0x7f3e31e44da0 "/home/kdesvn/kdelibs/solid/solid/devicemanager.cpp", 
    line=164) at /home/kdesvn/qt-copy/src/corelib/global/qglobal.cpp:1872
#10 0x00007f3e31dfc1be in Solid::DeviceManagerPrivate::_k_deviceAdded (this=0xa72620, udi=@0xa2a990) at /home/kdesvn/kdelibs/solid/solid/devicemanager.cpp:164
#11 0x00007f3e31dfc7ad in Solid::DeviceManagerPrivate::qt_metacall (this=0xa72620, _c=QMetaObject::InvokeMetaMethod, _id=0, _a=0x7fff3f2e95b0)
    at /tmp/kdesvn-build/kdelibs/solid/solid/devicemanager_p.moc:72
#12 0x00007f3e3562ac3f in QMetaObject::activate (sender=0x9daac0, from_signal_index=4, to_signal_index=4, argv=0x7fff3f2e95b0)
    at /home/kdesvn/qt-copy/src/corelib/kernel/qobject.cpp:3028
#13 0x00007f3e3562af29 in QMetaObject::activate (sender=0x9daac0, m=0x7f3e3206bd20, local_signal_index=0, argv=0x7fff3f2e95b0)
    at /home/kdesvn/qt-copy/src/corelib/kernel/qobject.cpp:3101
#14 0x00007f3e31e0fa27 in Solid::Ifaces::DeviceManager::deviceAdded (this=0x9daac0, _t1=@0xa2a990) at /tmp/kdesvn-build/kdelibs/solid/solid/ifaces/devicemanager.moc:79
#15 0x00007f3e31e309dc in Solid::Backends::Hal::HalManager::slotDeviceAdded (this=0x9daac0, udi=@0xa2a990)
    at /home/kdesvn/kdelibs/solid/solid/backends/hal/halmanager.cpp:213
#16 0x00007f3e31e30a49 in Solid::Backends::Hal::HalManager::qt_metacall (this=0x9daac0, _c=QMetaObject::InvokeMetaMethod, _id=0, _a=0x7fff3f2e9660)
    at /tmp/kdesvn-build/kdelibs/solid/solid/backends/hal/halmanager.moc:70
#17 0x00007f3e359509ae in QDBusConnectionPrivate::deliverCall (this=0x9491b0, object=0x9daac0, msg=@0xd44658, metaTypes=@0xd44660, slotIdx=6)
    at /home/kdesvn/qt-copy/src/dbus/qdbusintegrator.cpp:849
#18 0x00007f3e35958e6f in QDBusCallDeliveryEvent::placeMetaCall (this=0xd44610, object=0x9daac0) at /home/kdesvn/qt-copy/src/dbus/qdbusintegrator_p.h:130
#19 0x00007f3e3562f1a8 in QObject::event (this=0x9daac0, e=0xd44610) at /home/kdesvn/qt-copy/src/corelib/kernel/qobject.cpp:1152
#20 0x00007f3e34278bef in QApplicationPrivate::notify_helper (this=0x7f2c60, receiver=0x9daac0, e=0xd44610) at /home/kdesvn/qt-copy/src/gui/kernel/qapplication.cpp:3803
#21 0x00007f3e34278f11 in QApplication::notify (this=0x7fff3f2ea630, receiver=0x9daac0, e=0xd44610) at /home/kdesvn/qt-copy/src/gui/kernel/qapplication.cpp:3393
#22 0x00007f3e364a1b9e in KApplication::notify (this=0x7fff3f2ea630, receiver=0x9daac0, event=0xd44610) at /home/kdesvn/kdelibs/kdeui/kernel/kapplication.cpp:307
#23 0x00007f3e356175d4 in QCoreApplication::notifyInternal (this=0x7fff3f2ea630, receiver=0x9daac0, event=0xd44610)
    at /home/kdesvn/qt-copy/src/corelib/kernel/qcoreapplication.cpp:583
#24 0x00007f3e3561b915 in QCoreApplication::sendEvent (receiver=0x9daac0, event=0xd44610) at /home/kdesvn/qt-copy/src/corelib/kernel/qcoreapplication.h:209
#25 0x00007f3e3561a7ad in QCoreApplicationPrivate::sendPostedEvents (receiver=0x0, event_type=0, data=0x7c31e0)
    at /home/kdesvn/qt-copy/src/corelib/kernel/qcoreapplication.cpp:1195
#26 0x00007f3e3561a976 in QCoreApplication::sendPostedEvents (receiver=0x0, event_type=0) at /home/kdesvn/qt-copy/src/corelib/kernel/qcoreapplication.cpp:1091
#27 0x00007f3e35647947 in QCoreApplication::sendPostedEvents () at /home/kdesvn/qt-copy/src/corelib/kernel/qcoreapplication.h:214
#28 0x00007f3e35646175 in postEventSourceDispatch (s=0x7f63c0) at /home/kdesvn/qt-copy/src/corelib/kernel/qeventdispatcher_glib.cpp:205
#29 0x00007f3e306a8d3b in g_main_context_dispatch () from /usr/lib/libglib-2.0.so.0
#30 0x00007f3e306ac50d in ?? () from /usr/lib/libglib-2.0.so.0
#31 0x00007f3e306ac6cb in g_main_context_iteration () from /usr/lib/libglib-2.0.so.0
#32 0x00007f3e35646c54 in QEventDispatcherGlib::processEvents (this=0x7f2bd0, flags=@0x7fff3f2ea3c0) at /home/kdesvn/qt-copy/src/corelib/kernel/qeventdispatcher_glib.cpp:319
#33 0x00007f3e34318547 in QGuiEventDispatcherGlib::processEvents (this=0x7f2bd0, flags=@0x7fff3f2ea420) at /home/kdesvn/qt-copy/src/gui/kernel/qguieventdispatcher_glib.cpp:198
#34 0x00007f3e35616438 in QEventLoop::processEvents (this=0x7fff3f2ea4d0, flags=@0x7fff3f2ea480) at /home/kdesvn/qt-copy/src/corelib/kernel/qeventloop.cpp:143
#35 0x00007f3e35616633 in QEventLoop::exec (this=0x7fff3f2ea4d0, flags=@0x7fff3f2ea4e0) at /home/kdesvn/qt-copy/src/corelib/kernel/qeventloop.cpp:190
#36 0x00007f3e3561aa82 in QCoreApplication::exec () at /home/kdesvn/qt-copy/src/corelib/kernel/qcoreapplication.cpp:845
#37 0x00007f3e34277f90 in QApplication::exec () at /home/kdesvn/qt-copy/src/gui/kernel/qapplication.cpp:3331
#38 0x00007f3e36ebf753 in kdemain (argc=1, argv=0x7fff3f2ea9c8) at /home/kdesvn/kdelibs/kded/kded.cpp:922
#39 0x0000000000400851 in main (argc=1, argv=0x7fff3f2ea9c8) at /tmp/kdesvn-build/kdelibs/kded/kded4_dummy.cpp:3

Some poking around:

(gdb) up
#10 0x00007f3e31dfc1be in Solid::DeviceManagerPrivate::_k_deviceAdded (
    this=0xa72620, udi=@0xa2a990)
    at /home/kdesvn/kdelibs/solid/solid/devicemanager.cpp:164
164             Q_ASSERT(dev->backendObject()==0);
(gdb) p dev
$1 = (class Solid::DevicePrivate *) 0xd41bb0
(gdb) p dev->backendObject()
[Switching to Thread 0x7f3e372b56f0 (LWP 6046)]
$2 = (class Solid::Ifaces::Device *) 0xc35e00
(gdb) p * $2
$3 = {<QObject> = {_vptr.QObject = 0x7f3e32078270, static staticMetaObject = {
      d = {superdata = 0x0, stringdata = 0x7f3e356aea00 "QObject",
        data = 0x7f3e356ae960, extradata = 0x0}}, d_ptr = 0xbbc530,
    static staticQtMetaObject = {d = {superdata = 0x0,
        stringdata = 0x7f3e356bb3a0 "Qt", data = 0x7f3e356b9f60,
        extradata = 0x0}}}, static staticMetaObject = {d = {
      superdata = 0x7f3e35924500,
      stringdata = 0x7f3e31e48e30 "Solid::Ifaces::Device",
      data = 0x7f3e31e48e00, extradata = 0x0}}}
(gdb)

/home/kdesvn/kdelibs/solid/solid/devicemanager.cpp is SVN r906916.