backtrace from attachment:
Application: KWin (kwin), signal: Segmentation fault
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
[Current thread is 1 (Thread 0x7ff907e9b7c0 (LWP 30363))]

Thread 2 (Thread 0x7ff8e9f7b700 (LWP 30370)):
#0  pthread_cond_wait@@GLIBC_2.3.2 () at ../nptl/sysdeps/unix/sysv/linux/x86_64/pthread_cond_wait.S:162
#1  0x00007ff904170212 in ?? () from /usr/lib/x86_64-linux-gnu/libQtScript.so.4
#2  0x00007ff904170249 in ?? () from /usr/lib/x86_64-linux-gnu/libQtScript.so.4
#3  0x00007ff90391c354 in ?? () from /usr/lib/nvidia-current/libGL.so.1
#4  0x00007ff8fbef9e9a in start_thread (arg=0x7ff8e9f7b700) at pthread_create.c:308
#5  0x00007ff9076b674d in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:112
#6  0x0000000000000000 in ?? ()

Thread 1 (Thread 0x7ff907e9b7c0 (LWP 30363)):
[KCrash Handler]
#6  0x0000000000c60a40 in ?? ()
#7  0x00007ff907a7785e in KWin::TabBox::ImageProvider::requestPixmap (this=0xf7c8f0, id=..., size=0x7fff4bcbfc30, requestedSize=...) at ../../kwin/tabbox/declarative.cpp:79
#8  0x00007ff906456e80 in QDeclarativeEnginePrivate::getPixmapFromProvider (this=0xed69e0, url=..., size=0x7fff4bcbfc30, req_size=...) at qml/qdeclarativeengine.cpp:838
#9  0x00007ff90639ddb8 in createPixmapDataSync (ok=<synthetic pointer>, requestSize=..., url=..., engine=0xed46b0) at util/qdeclarativepixmapcache.cpp:828
#10 QDeclarativePixmap::load (this=0xf466e8, engine=0xed46b0, url=..., requestSize=..., options=...) at util/qdeclarativepixmapcache.cpp:1029
#11 0x00007ff9063e786d in QDeclarativeImageBase::load (this=0xf46450) at graphicsitems/qdeclarativeimagebase.cpp:205
#12 0x00007ff90654edc2 in QDeclarativeImageBase::qt_metacall (this=0xf46450, _c=QMetaObject::WriteProperty, _id=1, _a=0x7fff4bcbfed0) at .moc/release-shared/moc_qdeclarativeimagebase_p.cpp:184
#13 0x00007ff90654e8b5 in QDeclarativeImage::qt_metacall (this=0xf46450, _c=QMetaObject::WriteProperty, _id=<optimized out>, _a=0x7fff4bcbfed0) at .moc/release-shared/moc_qdeclarativeimage_p.cpp:112
#14 0x00007ff906466d7f in QDeclarativePropertyPrivate::write (object=0xf46450, property=..., value=..., context=0xf3ce80, flags=...) at qml/qdeclarativeproperty.cpp:1142
#15 0x00007ff906467531 in QDeclarativePropertyPrivate::writeValueProperty (this=0xf467c0, value=..., flags=...) at qml/qdeclarativeproperty.cpp:1088
#16 0x00007ff906467626 in QDeclarativePropertyPrivate::write (that=..., value=..., flags=...) at qml/qdeclarativeproperty.cpp:1343
#17 0x00007ff906460f29 in QDeclarativeBinding::update (this=0xf46840, flags=...) at qml/qdeclarativebinding.cpp:424
#18 0x00007ff90645fd3b in update (this=<optimized out>) at ../../include/QtDeclarative/private/../../../src/declarative/qml/qdeclarativebinding_p.h:177
#19 QDeclarativeBindingPrivate::emitValueChanged (this=<optimized out>) at qml/qdeclarativebinding.cpp:466
#20 0x00007ff90645ecbb in QDeclarativeExpression::qt_metacall (this=0xf46840, _c=QMetaObject::InvokeMetaMethod, _id=<optimized out>, _a=0x7fff4bcc04b0) at .moc/release-shared/moc_qdeclarativeexpression.cpp:94
#21 0x00007ff90655afa0 in QDeclarativeBinding::qt_metacall (this=0xf46840, _c=QMetaObject::InvokeMetaMethod, _id=<optimized out>, _a=0x7fff4bcc04b0) at .moc/release-shared/moc_qdeclarativebinding_p.cpp:86
#22 0x00007ff902de2af9 in QMetaObject::activate (sender=0xf3b740, m=<optimized out>, local_signal_index=<optimized out>, argv=0x0) at kernel/qobject.cpp:3566
#23 0x00007ff906434d70 in QDeclarativeListViewPrivate::updateCurrent (this=0xfe5130, modelIndex=2) at graphicsitems/qdeclarativelistview.cpp:1183
#24 0x00007ff906559381 in QDeclarativeListView::qt_metacall (this=0xf3b740, _c=QMetaObject::WriteProperty, _id=2, _a=0x7fff4bcc0740) at .moc/release-shared/moc_qdeclarativelistview_p.cpp:560
#25 0x00007ff90644abec in QDeclarativeVMEMetaObject::metaCall (this=0xfe6040, c=QMetaObject::WriteProperty, _id=69, a=0x7fff4bcc0740) at qml/qdeclarativevmemetaobject.cpp:673
#26 0x00007ff902dd7d56 in QMetaProperty::write (this=0x7fff4bcc0830, object=0xf3b740, value=...) at kernel/qmetaobject.cpp:2341
#27 0x00007ff902de85a3 in QObject::setProperty (this=0xf3b740, name=0x7ff907a9eca5 "currentIndex", value=...) at kernel/qobject.cpp:3725
#28 0x00007ff907a76c6b in KWin::TabBox::DeclarativeView::setCurrentIndex (this=<optimized out>, index=...) at ../../kwin/tabbox/declarative.cpp:180
#29 0x00007ff907a7dee0 in KWin::TabBox::TabBoxHandler::setCurrentIndex (this=0xa67980, index=...) at ../../kwin/tabbox/tabboxhandler.cpp:544
#30 0x00007ff907a703f2 in setCurrentIndex (notifyEffects=false, index=..., this=0xa68630) at ../../kwin/tabbox/tabbox.cpp:496
#31 KWin::TabBox::TabBox::setCurrentIndex (this=0xa68630, index=..., notifyEffects=false) at ../../kwin/tabbox/tabbox.cpp:492
#32 0x00007ff907a704c5 in KWin::TabBox::TabBox::nextPrev (this=0xa68630, next=<optimized out>) at ../../kwin/tabbox/tabbox.cpp:418
#33 0x00007ff907a704fd in KWin::TabBox::TabBox::walkThroughDesktops (this=0xa68630, forward=<optimized out>) at ../../kwin/tabbox/tabbox.cpp:956
#34 0x00007ff907a725af in KWin::TabBox::TabBox::keyPress (this=0xa68630, keyQt=83886081) at ../../kwin/tabbox/tabbox.cpp:1077
#35 0x00007ff9079ed6cd in KWin::Workspace::workspaceEvent (this=0x9956c0, e=0x7fff4bcc11b0) at ../../kwin/events.cpp:262
#36 0x00007ff9079defa8 in KWin::Application::x11EventFilter (this=0x7fff4bcc14c0, e=0x7fff4bcc11b0) at ../../kwin/main.cpp:359
#37 0x00007ff9021c09a5 in qt_x11EventFilter (ev=0x7fff4bcc11b0) at kernel/qapplication_x11.cpp:441
#38 qt_x11EventFilter (ev=0x7fff4bcc11b0) at kernel/qapplication_x11.cpp:429
#39 0x00007ff9021cfdb8 in QApplication::x11ProcessEvent (this=0x7fff4bcc14c0, event=0x7fff4bcc11b0) at kernel/qapplication_x11.cpp:3444
#40 0x00007ff9021fa8fa in QEventDispatcherX11::processEvents (this=0x874ad0, flags=...) at kernel/qeventdispatcher_x11.cpp:132
#41 0x00007ff902dcd2f2 in QEventLoop::processEvents (this=<optimized out>, flags=...) at kernel/qeventloop.cpp:149
#42 0x00007ff902dcd547 in QEventLoop::exec (this=0x7fff4bcc1400, flags=...) at kernel/qeventloop.cpp:204
#43 0x00007ff902dd25d7 in QCoreApplication::exec () at kernel/qcoreapplication.cpp:1148
#44 0x00007ff9079e1726 in kdemain (argc=<optimized out>, argv=<optimized out>) at ../../kwin/main.cpp:541
#45 0x00007ff9075e776d in __libc_start_main (main=0x400630 <main(int, char**)>, argc=3, ubp_av=0x7fff4bcc1be8, init=<optimized out>, fini=<optimized out>, rtld_fini=<optimized out>, stack_end=0x7fff4bcc1bd8) at libc-start.c:226
#46 0x0000000000400661 in _start ()

*** This bug has been marked as a duplicate of bug 285747 ***

Bugzilla search failed me. Happy to hear it's already fixed though. Now to get Ubuntu to merge the fix...

it's "fixed" by replacing switcher - ubuntu will unlikey backport that to 4.8