Bug 337850

this happens half the time here.
1) enable grouping always (not only when taskbar is full).
2) open firefox and firefox bookmarks library.
3) click on firefox entry in the taskbar.
http://i.imgur.com/Co1fRez.png
4) now drag 'New tab' over 'Library' and notice the Firefox icon.
5) If you do this fast enough while a third application is above firefox, plasma will crash. This appears to be reproduce-able once per kde session only.

This is the backtrace drkonqi showed although it is not enough. I thought i should report it anyway.
-- Backtrace:
Application: Plasma Desktop Shell (plasma-desktop), signal: Segmentation fault
Using host libthread_db library "/usr/lib/libthread_db.so.1".
[Current thread is 1 (Thread 0x7f930e2e07c0 (LWP 32733))]

Thread 3 (Thread 0x7f92e4f28700 (LWP 32734)):
#0  0x00007f930a369aaf in pthread_cond_wait@@GLIBC_2.3.2 () from /usr/lib/libpthread.so.0
#1  0x00007f9306bbed4a in ?? () from /usr/lib/libQtScript.so.4
#2  0x00007f9306bbed79 in ?? () from /usr/lib/libQtScript.so.4
#3  0x00007f930a3650f4 in start_thread () from /usr/lib/libpthread.so.0
#4  0x00007f930dbdb4bd in clone () from /usr/lib/libc.so.6

Thread 2 (Thread 0x7f925af4b700 (LWP 32735)):
#0  0x00007f930dbd281d in poll () from /usr/lib/libc.so.6
#1  0x00007f9300faad64 in ?? () from /usr/lib/libglib-2.0.so.0
#2  0x00007f9300faae6c in g_main_context_iteration () from /usr/lib/libglib-2.0.so.0
#3  0x00007f930a731507 in QEventDispatcherGlib::processEvents(QFlags<QEventLoop::ProcessEventsFlag>) () from /usr/lib/libQtCore.so.4
#4  0x00007f930a700de1 in QEventLoop::processEvents(QFlags<QEventLoop::ProcessEventsFlag>) () from /usr/lib/libQtCore.so.4
#5  0x00007f930a701145 in QEventLoop::exec(QFlags<QEventLoop::ProcessEventsFlag>) () from /usr/lib/libQtCore.so.4
#6  0x00007f930a5f58b9 in QThread::exec() () from /usr/lib/libQtCore.so.4
#7  0x00007f930a6e17c3 in ?? () from /usr/lib/libQtCore.so.4
#8  0x00007f930a5f813f in ?? () from /usr/lib/libQtCore.so.4
#9  0x00007f930a3650f4 in start_thread () from /usr/lib/libpthread.so.0
#10 0x00007f930dbdb4bd in clone () from /usr/lib/libc.so.6

Thread 1 (Thread 0x7f930e2e07c0 (LWP 32733)):
[KCrash Handler]
#5  0x00007f930c452ff0 in QDeclarativeError::isValid() const () from /usr/lib/libQtDeclarative.so.4
#6  0x00007f930c43b6eb in ?? () from /usr/lib/libQtDeclarative.so.4
#7  0x00007f930a7172eb in QMetaObject::activate(QObject*, QMetaObject const*, int, void**) () from /usr/lib/libQtCore.so.4
#8  0x00007f925f9a5c61 in ?? () from /usr/lib/kde4/imports/org/kde/qtextracomponents/libqtextracomponentsplugin.so
#9  0x00007f9309e1cc03 in QGraphicsItem::sceneEvent(QEvent*) () from /usr/lib/libQtGui.so.4
#10 0x00007f930c394464 in QDeclarativeItem::sceneEvent(QEvent*) () from /usr/lib/libQtDeclarative.so.4
#11 0x00007f9309e45ca7 in ?? () from /usr/lib/libQtGui.so.4
#12 0x00007f9309e4efc6 in QGraphicsScene::mouseMoveEvent(QGraphicsSceneMouseEvent*) () from /usr/lib/libQtGui.so.4
#13 0x00007f9309e5beaf in QGraphicsScene::event(QEvent*) () from /usr/lib/libQtGui.so.4
#14 0x00007f9309838b9c in QApplicationPrivate::notify_helper(QObject*, QEvent*) () from /usr/lib/libQtGui.so.4
#15 0x00007f930983f638 in QApplication::notify(QObject*, QEvent*) () from /usr/lib/libQtGui.so.4
#16 0x00007f930b41f42a in KApplication::notify(QObject*, QEvent*) () from /usr/lib/libkdeui.so.5
#17 0x00007f930a7022ad in QCoreApplication::notifyInternal(QObject*, QEvent*) () from /usr/lib/libQtCore.so.4
#18 0x00007f9309e72188 in QGraphicsViewPrivate::mouseMoveEventHandler(QMouseEvent*) () from /usr/lib/libQtGui.so.4
#19 0x00007f9309e7321a in QGraphicsView::mouseMoveEvent(QMouseEvent*) () from /usr/lib/libQtGui.so.4
#20 0x00007f930988bdc8 in QWidget::event(QEvent*) () from /usr/lib/libQtGui.so.4
#21 0x00007f9309c4c83e in QFrame::event(QEvent*) () from /usr/lib/libQtGui.so.4
#22 0x00007f9309e749cd in QGraphicsView::viewportEvent(QEvent*) () from /usr/lib/libQtGui.so.4
#23 0x00007f930a702416 in QCoreApplicationPrivate::sendThroughObjectEventFilters(QObject*, QEvent*) () from /usr/lib/libQtCore.so.4
#24 0x00007f9309838b7c in QApplicationPrivate::notify_helper(QObject*, QEvent*) () from /usr/lib/libQtGui.so.4
#25 0x00007f930983f79f in QApplication::notify(QObject*, QEvent*) () from /usr/lib/libQtGui.so.4
#26 0x00007f930b41f42a in KApplication::notify(QObject*, QEvent*) () from /usr/lib/libkdeui.so.5
#27 0x00007f930a7022ad in QCoreApplication::notifyInternal(QObject*, QEvent*) () from /usr/lib/libQtCore.so.4
#28 0x00007f930983edcf in QApplicationPrivate::sendMouseEvent(QWidget*, QMouseEvent*, QWidget*, QWidget*, QWidget**, QPointer<QWidget>&, bool) () from /usr/lib/libQtGui.so.4
#29 0x00007f93098b610d in ?? () from /usr/lib/libQtGui.so.4
#30 0x00007f93098b4732 in QApplication::x11ProcessEvent(_XEvent*) () from /usr/lib/libQtGui.so.4
#31 0x00007f93098dcaf2 in ?? () from /usr/lib/libQtGui.so.4
#32 0x00007f9300faab84 in g_main_context_dispatch () from /usr/lib/libglib-2.0.so.0
#33 0x00007f9300faadc8 in ?? () from /usr/lib/libglib-2.0.so.0
#34 0x00007f9300faae6c in g_main_context_iteration () from /usr/lib/libglib-2.0.so.0
#35 0x00007f930a7314ed in QEventDispatcherGlib::processEvents(QFlags<QEventLoop::ProcessEventsFlag>) () from /usr/lib/libQtCore.so.4
#36 0x00007f93098dcbb6 in ?? () from /usr/lib/libQtGui.so.4
#37 0x00007f930a700de1 in QEventLoop::processEvents(QFlags<QEventLoop::ProcessEventsFlag>) () from /usr/lib/libQtCore.so.4
#38 0x00007f930a701145 in QEventLoop::exec(QFlags<QEventLoop::ProcessEventsFlag>) () from /usr/lib/libQtCore.so.4
#39 0x00007f930a7066f7 in QCoreApplication::exec() () from /usr/lib/libQtCore.so.4
#40 0x00007f930dee0452 in kdemain () from /usr/lib/libkdeinit4_plasma-desktop.so
#41 0x00007f930db12000 in __libc_start_main () from /usr/lib/libc.so.6
#42 0x000000000040070e in _start ()