Here using:

Qt: 4.5.1 (qt-copy  958974)
KDE: 4.2.85 (KDE 4.2.85 (KDE 4.3 Beta1))
kdelibs svn rev. 967040 / kdebase svn rev. 967041
on ArchLinux i686 - Kernel 2.6.29.2

I can reproduce the crash with the provided steps.
However, the "Overwrite" file dialog didn't appear. 
It only appeared a message in the Dolphin statusbar saying "The file X already exists"

And the notification can be cancelled and removed (this was fixed from the Plasma side)

I guess it is trying to kill a job which doesn't exist. I wonder if this is a kdelibs bug.

Backtrace:
---

Application: Dolphin (dolphin), signal: Segmentation fault
[KCrash Handler]
#6  0x0070004f in ?? ()
#7  0xb7317d32 in KJob::kill (this=0x874d598, verbosity=KJob::EmitResult) at /home/kde-devel/kde/src/KDE/kdelibs/kdecore/jobs/kjob.cpp:106
#8  0xb76c9a7f in KUiServerJobTracker::Private::_k_killJob (this=0x8795438) at /home/kde-devel/kde/src/KDE/kdelibs/kdeui/jobs/kuiserverjobtracker.cpp:58
#9  0xb76c9af9 in KUiServerJobTracker::qt_metacall (this=0x87452e0, _c=QMetaObject::InvokeMetaMethod, _id=25, _a=0xbfba6a98) at /home/kde-devel/kde/build/KDE/kdelibs/kdeui/kuiserverjobtracker.moc:94
#10 0xb67e3b3b in QMetaObject::activate (sender=0x87393c8, from_signal_index=5, to_signal_index=5, argv=0x0) at kernel/qobject.cpp:3117
#11 0xb67e4f07 in QMetaObject::activate (sender=0x87393c8, m=0xb78d3a64, local_signal_index=0, argv=0x0) at kernel/qobject.cpp:3194
#12 0xb783e127 in OrgKdeJobViewInterface::cancelRequested (this=0x87393c8) at /home/kde-devel/kde/build/KDE/kdelibs/kdeui/jobviewiface.moc:113
#13 0xb783e52b in OrgKdeJobViewInterface::qt_metacall (this=0x87393c8, _c=QMetaObject::InvokeMetaMethod, _id=0, _a=0xbfba6cc0) at /home/kde-devel/kde/build/KDE/kdelibs/kdeui/jobviewiface.moc:90
#14 0xb72020be in QDBusConnectionPrivate::deliverCall (this=0x82f9810, object=0x87393c8, msg=@0x87398ac, metaTypes=@0x87398b0, slotIdx=5) at qdbusintegrator.cpp:891
#15 0xb720d4d8 in QDBusCallDeliveryEvent::placeMetaCall (this=0x8739880, object=0x87393c8) at qdbusintegrator_p.h:101
#16 0xb67e19ef in QObject::event (this=0x87393c8, e=0x8739880) at kernel/qobject.cpp:1115
#17 0xb69f51d5 in QApplicationPrivate::notify_helper (this=0x83011c8, receiver=0x87393c8, e=0x8739880) at kernel/qapplication.cpp:4057
#18 0xb69f5589 in QApplication::notify (this=0xbfba764c, receiver=0x87393c8, e=0x8739880) at kernel/qapplication.cpp:3604
#19 0xb76cf24a in KApplication::notify (this=0xbfba764c, receiver=0x87393c8, event=0x8739880) at /home/kde-devel/kde/src/KDE/kdelibs/kdeui/kernel/kapplication.cpp:307
#20 0xb67cd6cb in QCoreApplication::notifyInternal (this=0xbfba764c, receiver=0x87393c8, event=0x8739880) at kernel/qcoreapplication.cpp:606
#21 0xb69f1bbd in QCoreApplication::sendEvent (receiver=0x87393c8, event=0x8739880) at ../../include/QtCore/../../src/corelib/kernel/qcoreapplication.h:213
#22 0xb67cdbb3 in QCoreApplicationPrivate::sendPostedEvents (receiver=0x0, event_type=0, data=0x82c9998) at kernel/qcoreapplication.cpp:1244
#23 0xb67cdd6f in QCoreApplication::sendPostedEvents (receiver=0x0, event_type=0) at kernel/qcoreapplication.cpp:1140
#24 0xb6aa45ec in QCoreApplication::sendPostedEvents () at ../../include/QtCore/../../src/corelib/kernel/qcoreapplication.h:218
#25 0xb67fd035 in postEventSourceDispatch (s=0x82c94c0) at kernel/qeventdispatcher_glib.cpp:209
#26 0xb5fe9718 in g_main_context_dispatch () from /usr/lib/libglib-2.0.so.0
#27 0xb5fecc7b in g_main_context_iterate () from /usr/lib/libglib-2.0.so.0
#28 0xb5fecdf8 in g_main_context_iteration () from /usr/lib/libglib-2.0.so.0
#29 0xb67fc41e in QEventDispatcherGlib::processEvents (this=0x83099c0, flags={i = -1078299384}) at kernel/qeventdispatcher_glib.cpp:324
#30 0xb6aa2dd8 in QGuiEventDispatcherGlib::processEvents (this=0x83099c0, flags={i = -1078299336}) at kernel/qguieventdispatcher_glib.cpp:202
#31 0xb67ca6b0 in QEventLoop::processEvents (this=0xbfba75c0, flags={i = -1078299264}) at kernel/qeventloop.cpp:149
#32 0xb67ca90a in QEventLoop::exec (this=0xbfba75c0, flags={i = -1078299192}) at kernel/qeventloop.cpp:196
#33 0xb67cdea3 in QCoreApplication::exec () at kernel/qcoreapplication.cpp:888
#34 0xb69f4eee in QApplication::exec () at kernel/qapplication.cpp:3526
#35 0x0807a6af in main (argc=5, argv=0xbfba7824) at /home/kde-devel/kde/src/KDE/kdebase/apps/dolphin/src/main.cpp:94