Application: kwin (4.11.3)
KDE Platform Version: 4.11.3 (Compiled from sources)
Qt Version: 4.8.6
Operating System: Linux 3.12.0-aufs x86_64
Distribution: "Gentoo Base System release 2.2"

-- Information about the crash:
- What I was doing when the application crashed:
→ Activated 'Desktop Grid' effect using the defined shortcut Ctrl+Space
→ Dragged a LibreOffice Calc window from desktop#1 to desktop#2
→ Started dragging another LibreOffice Calc window from desktop#1 to desktop#3
→ kwin crashed
Probably relevant: there was another LibreOffice Calc window left on desktop#1 and 1 Firefox window. No other windows were present on any other desktop.

The configuration of the 'Desktop Grid' effect:
Zoom Duration: Default
Border width: 10
Desktop name alignment: Center
Layout mode: Automatic
[x] Use Present Windows effect to layout the windows

Unfortunately, I wasn't able yet to reproduce this crash - if the BT doesn't provide sufficent information to work on this bug without knowing how to exactly reproduce it, feel free to close it.

The crash does not seem to be reproducible.

-- Backtrace:
Application: KWin (kwin), signal: Segmentation fault
Using host libthread_db library "/lib64/libthread_db.so.1".
[Current thread is 1 (Thread 0x7f6bca7e17c0 (LWP 4495))]

Thread 2 (Thread 0x7f6b9fddb700 (LWP 4507)):
#0  pthread_cond_wait () at ../nptl/sysdeps/unix/sysv/linux/x86_64/pthread_cond_wait.S:162
#1  0x00007f6bc91e2807 in QTWTF::TCMalloc_PageHeap::scavengerThread (this=0x7f6bc94f0660 <QTWTF::pageheap_memory>) at ../3rdparty/javascriptcore/JavaScriptCore/wtf/FastMalloc.cpp:2359
#2  0x00007f6bc91e2839 in QTWTF::TCMalloc_PageHeap::runScavengerThread (context=<optimized out>) at ../3rdparty/javascriptcore/JavaScriptCore/wtf/FastMalloc.cpp:1464
#3  0x00007f6bc49b7ec6 in start_thread (arg=0x7f6b9fddb700) at pthread_create.c:305
#4  0x00007f6bc9fe789d in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:115

Thread 1 (Thread 0x7f6bca7e17c0 (LWP 4495)):
[KCrash Handler]
#6  QObject::property (this=0x18f8360, name=0x7f6bc9ad20e1 "geometry") at kernel/qobject.cpp:3752
#7  0x00007f6bc9ac050d in KWin::EffectWindow::geometry (this=<optimized out>) at /var/tmp/portage/kde-base/kwin-4.11.3/work/kwin-4.11.3/kwin/libkwineffects/kwineffects.cpp:690
#8  0x00007f6bc9ac46c7 in KWin::WindowMotionManager::transformedGeometry (this=this@entry=0x1ba5c70, w=0x18f8370) at /var/tmp/portage/kde-base/kwin-4.11.3/work/kwin-4.11.3/kwin/libkwineffects/kwineffects.cpp:1609
#9  0x00007f6bc9ac4829 in KWin::WindowMotionManager::windowAtPoint (this=0x1ba5c70, point=..., useStackingOrder=<optimized out>) at /var/tmp/portage/kde-base/kwin-4.11.3/work/kwin-4.11.3/kwin/libkwineffects/kwineffects.cpp:1652
#10 0x00007f6b1cc12ac0 in KWin::DesktopGridEffect::windowAt (this=this@entry=0xb5bd90, pos=...) at /var/tmp/portage/kde-base/kwin-4.11.3/work/kwin-4.11.3/kwin/effects/desktopgrid/desktopgrid.cpp:884
#11 0x00007f6b1cc171a1 in KWin::DesktopGridEffect::windowInputMouseEvent (this=0xb5bd90, e=0x7fffaacf1140) at /var/tmp/portage/kde-base/kwin-4.11.3/work/kwin-4.11.3/kwin/effects/desktopgrid/desktopgrid.cpp:591
#12 0x00007f6bca399513 in KWin::EffectsHandlerImpl::checkInputWindowEvent (this=<optimized out>, e=0x7fffaacf1870) at /var/tmp/portage/kde-base/kwin-4.11.3/work/kwin-4.11.3/kwin/effects.cpp:1238
#13 0x00007f6bca32c205 in KWin::Workspace::workspaceEvent (this=0xcbedb0, e=0x7fffaacf1870) at /var/tmp/portage/kde-base/kwin-4.11.3/work/kwin-4.11.3/kwin/events.cpp:113
#14 0x00007f6bca321e48 in KWin::Application::x11EventFilter (this=0x7fffaacf1dc0, e=0x7fffaacf1870) at /var/tmp/portage/kde-base/kwin-4.11.3/work/kwin-4.11.3/kwin/main.cpp:422
#15 0x00007f6bc3f422ec in qt_x11EventFilter (ev=0x7fffaacf1870) at kernel/qapplication_x11.cpp:436
#16 qt_x11EventFilter (ev=0x7fffaacf1870) at kernel/qapplication_x11.cpp:424
#17 0x00007f6bc3f4dd6b in QApplication::x11ProcessEvent (this=0x7fffaacf1dc0, event=0x7fffaacf1870) at kernel/qapplication_x11.cpp:3354
#18 0x00007f6bc3f77f11 in QEventDispatcherX11::processEvents (this=0xabbfe0, flags=...) at kernel/qeventdispatcher_x11.cpp:132
#19 0x00007f6bc4d4b7df in QEventLoop::processEvents (this=this@entry=0x7fffaacf1ad0, flags=...) at kernel/qeventloop.cpp:149
#20 0x00007f6bc4d4ba68 in QEventLoop::exec (this=0x7fffaacf1ad0, flags=...) at kernel/qeventloop.cpp:204
#21 0x00007f6bc4d50e48 in QCoreApplication::exec () at kernel/qcoreapplication.cpp:1225
#22 0x00007f6bca321719 in kdemain (argc=1, argv=0x7fffaacf1f08) at /var/tmp/portage/kde-base/kwin-4.11.3/work/kwin-4.11.3/kwin/main.cpp:597
#23 0x00007f6bc9f2160d in __libc_start_main (main=0x4007c0 <main(int, char**)>, argc=1, ubp_av=0x7fffaacf1f08, init=<optimized out>, fini=<optimized out>, rtld_fini=<optimized out>, stack_end=0x7fffaacf1ef8) at libc-start.c:226
#24 0x00000000004007f1 in _start ()

Reported using DrKonqi