While debugging things in bug 352548 and changing settings in System Settings, plasmashell froze, but I guess it could happen in any app:

#0  QCoreApplication::removePostedEvents (receiver=0x576efca0, eventType=eventType@entry=0) at kernel/qcoreapplication.cpp:1609
#1  0x00007f53fc9c31f1 in QObjectPrivate::~QObjectPrivate (this=0x2be3a7b0, __in_chrg=<optimized out>) at kernel/qobject.cpp:234
#2  0x00007f53fc9c3329 in QObjectPrivate::~QObjectPrivate (this=0x2be3a7b0, __in_chrg=<optimized out>) at kernel/qobject.cpp:245
#3  0x00007f53fc9cdfb2 in QScopedPointerDeleter<QObjectData>::cleanup (pointer=<optimized out>) at ../../include/QtCore/../../src/corelib/tools/qscopedpointer.h:54
#4  QScopedPointer<QObjectData, QScopedPointerDeleter<QObjectData> >::~QScopedPointer (this=0x576efca8, __in_chrg=<optimized out>) at ../../include/QtCore/../../src/corelib/tools/qscopedpointer.h:101
#5  QObject::~QObject (this=<optimized out>, __in_chrg=<optimized out>) at kernel/qobject.cpp:882
#6  0x00007f53f1559d62 in Breeze::AnimationData::~AnimationData (this=0x576efca0, __in_chrg=<optimized out>)
    at /var/tmp/portage/kde-plasma/breeze-5.4.0/work/breeze-5.4.0-kf5/kstyle/../../breeze-5.4.0/kstyle/animations/breezeanimationdata.h:51
#7  Breeze::GenericData::~GenericData (this=0x576efca0, __in_chrg=<optimized out>)
    at /var/tmp/portage/kde-plasma/breeze-5.4.0/work/breeze-5.4.0-kf5/kstyle/../../breeze-5.4.0/kstyle/animations/breezegenericdata.h:48
#8  Breeze::WidgetStateData::~WidgetStateData (this=0x576efca0, __in_chrg=<optimized out>)
    at /var/tmp/portage/kde-plasma/breeze-5.4.0/work/breeze-5.4.0-kf5/kstyle/../../breeze-5.4.0/kstyle/animations/breezewidgetstatedata.h:45
#9  Breeze::EnableData::~EnableData (this=0x576efca0, __in_chrg=<optimized out>)
    at /var/tmp/portage/kde-plasma/breeze-5.4.0/work/breeze-5.4.0-kf5/kstyle/../../breeze-5.4.0/kstyle/animations/breezeenabledata.h:43
#10 Breeze::EnableData::~EnableData (this=0x576efca0, __in_chrg=<optimized out>)
    at /var/tmp/portage/kde-plasma/breeze-5.4.0/work/breeze-5.4.0-kf5/kstyle/../../breeze-5.4.0/kstyle/animations/breezeenabledata.h:43
#11 0x00007f53fc9c3c5b in QObjectPrivate::deleteChildren (this=this@entry=0x37d515d0) at kernel/qobject.cpp:1950
#12 0x00007f53fc9cdf6c in QObject::~QObject (this=<optimized out>, __in_chrg=<optimized out>) at kernel/qobject.cpp:1030
#13 0x00007f53f155b2ed in Breeze::BaseEngine::~BaseEngine (this=0x46f48520, __in_chrg=<optimized out>)
    at /var/tmp/portage/kde-plasma/breeze-5.4.0/work/breeze-5.4.0-kf5/kstyle/../../breeze-5.4.0/kstyle/animations/breezebaseengine.h:51
#14 Breeze::WidgetStateEngine::~WidgetStateEngine (this=0x46f48520, __in_chrg=<optimized out>)
    at /var/tmp/portage/kde-plasma/breeze-5.4.0/work/breeze-5.4.0-kf5/kstyle/../../breeze-5.4.0/kstyle/animations/breezewidgetstateengine.h:46
#15 Breeze::WidgetStateEngine::~WidgetStateEngine (this=0x46f48520, __in_chrg=<optimized out>)
    at /var/tmp/portage/kde-plasma/breeze-5.4.0/work/breeze-5.4.0-kf5/kstyle/../../breeze-5.4.0/kstyle/animations/breezewidgetstateengine.h:46
#16 0x00007f53fc9c3c5b in QObjectPrivate::deleteChildren (this=this@entry=0x37d51ea0) at kernel/qobject.cpp:1950
#17 0x00007f53fc9cdf6c in QObject::~QObject (this=<optimized out>, __in_chrg=<optimized out>) at kernel/qobject.cpp:1030
#18 0x00007f53f1559f1f in Breeze::Animations::~Animations (this=0x5aa640c0, __in_chrg=<optimized out>)
    at /var/tmp/portage/kde-plasma/breeze-5.4.0/work/breeze-5.4.0-kf5/kstyle/../../breeze-5.4.0/kstyle/animations/breezeanimations.h:52
#19 Breeze::Animations::~Animations (this=0x5aa640c0, __in_chrg=<optimized out>)
    at /var/tmp/portage/kde-plasma/breeze-5.4.0/work/breeze-5.4.0-kf5/kstyle/../../breeze-5.4.0/kstyle/animations/breezeanimations.h:52
#20 0x00007f53fc9c3c5b in QObjectPrivate::deleteChildren (this=this@entry=0x47717ff0) at kernel/qobject.cpp:1950
#21 0x00007f53fc9cdf6c in QObject::~QObject (this=<optimized out>, __in_chrg=<optimized out>) at kernel/qobject.cpp:1030
#22 0x00007f53f15390a9 in Breeze::Style::~Style (this=0x52d4fdc0, __in_chrg=<optimized out>) at /var/tmp/portage/kde-plasma/breeze-5.4.0/work/breeze-5.4.0/kstyle/breezestyle.cpp:206
#23 0x00007f53fd23f6e4 in QApplication::setStyle (style=<optimized out>) at kernel/qapplication.cpp:1263
#24 0x00007f53fd23fe66 in QApplication::setStyle (style=...) at kernel/qapplication.cpp:1295
#25 0x00007f53f1cd77a3 in KHintsSettings::slotNotifyChange (this=0xc30cd0, type=<optimized out>, arg=<optimized out>)
    at /var/tmp/portage/kde-frameworks/frameworkintegration-5.13.0/work/frameworkintegration-5.13.0/src/platformtheme/khintssettings.cpp:274
#26 0x00007f53f1ce1fd3 in KHintsSettings::qt_metacall (this=0xc30cd0, _c=QMetaObject::InvokeMetaMethod, _id=3, _a=0x7fff9640f220)
    at /var/tmp/portage/kde-frameworks/frameworkintegration-5.13.0/work/frameworkintegration-5.13.0_build/src/platformtheme/moc_khintssettings.cpp:116
#27 0x00007f53fd07651f in QDBusConnectionPrivate::deliverCall (this=0xc523c0, object=0xc30cd0, msg=..., metaTypes=..., slotIdx=8) at qdbusintegrator.cpp:993
#28 0x00007f53fc9c6296 in QObject::event (this=0xc30cd0, e=<optimized out>) at kernel/qobject.cpp:1245
#29 0x00007f53fd23d86c in QApplicationPrivate::notify_helper (this=this@entry=0xc00b70, receiver=receiver@entry=0xc30cd0, e=e@entry=0x4b8897c0) at kernel/qapplication.cpp:3720
#30 0x00007f53fd242680 in QApplication::notify (this=0x7fff9640fa50, receiver=0xc30cd0, e=0x4b8897c0) at kernel/qapplication.cpp:3503
#31 0x00007f53fc99495d in QCoreApplication::notifyInternal (this=0x7fff9640fa50, receiver=0xc30cd0, event=event@entry=0x4b8897c0) at kernel/qcoreapplication.cpp:935
#32 0x00007f53fc99689f in QCoreApplication::sendEvent (event=0x4b8897c0, receiver=<optimized out>) at kernel/qcoreapplication.h:228
#33 QCoreApplicationPrivate::sendPostedEvents (receiver=receiver@entry=0x0, event_type=event_type@entry=0, data=0xbf61f0) at kernel/qcoreapplication.cpp:1552
#34 0x00007f53fc996f08 in QCoreApplication::sendPostedEvents (receiver=receiver@entry=0x0, event_type=event_type@entry=0) at kernel/qcoreapplication.cpp:1410
#35 0x00007f53fc9eeae3 in postEventSourceDispatch (s=0xc33d00) at kernel/qeventdispatcher_glib.cpp:271
#36 0x00007f53f95479c4 in g_main_dispatch (context=0x7f53ec002450) at /var/tmp/portage/dev-libs/glib-2.44.1/work/glib-2.44.1/glib/gmain.c:3122
#37 g_main_context_dispatch (context=context@entry=0x7f53ec002450) at /var/tmp/portage/dev-libs/glib-2.44.1/work/glib-2.44.1/glib/gmain.c:3737
#38 0x00007f53f9547c18 in g_main_context_iterate (context=context@entry=0x7f53ec002450, block=block@entry=1, dispatch=dispatch@entry=1, self=<optimized out>)
    at /var/tmp/portage/dev-libs/glib-2.44.1/work/glib-2.44.1/glib/gmain.c:3808
#39 0x00007f53f9547cbc in g_main_context_iteration (context=0x7f53ec002450, may_block=1) at /var/tmp/portage/dev-libs/glib-2.44.1/work/glib-2.44.1/glib/gmain.c:3869
#40 0x00007f53fc9edf2c in QEventDispatcherGlib::processEvents (this=0xc34090, flags=...) at kernel/qeventdispatcher_glib.cpp:418
#41 0x00007f53fc9927eb in QEventLoop::exec (this=this@entry=0x7fff9640f8c0, flags=..., flags@entry=...) at kernel/qeventloop.cpp:204
#42 0x00007f53fc99a23f in QCoreApplication::exec () at kernel/qcoreapplication.cpp:1188
#43 0x00007f53fccb205c in QGuiApplication::exec () at kernel/qguiapplication.cpp:1507
#44 0x00007f53fd239f25 in QApplication::exec () at kernel/qapplication.cpp:2956
#45 0x000000000043218b in main (argc=2, argv=<optimized out>) at /var/tmp/portage/kde-plasma/plasma-workspace-5.4.0/work/plasma-workspace-5.4.0/shell/main.cpp:176

The backtrace didn't change even after detaching gdb for 60 seconds and then re-attaching.  I was only able to duplicate once (so twice) out of 20 different tries, so it seems to be a race that is not common but does happen.

Reproducible: Sometimes

Steps to Reproduce:
1. use KDE Plasma 5
2. change settings that trigger KHintsSettings::slotNotifyChange


Actual Results:  
Plasma Shell locked up.

Expected Results:  
The settings should have been changed without a lock up.