Ubuntu 8.10 x86_64
Same thing was seen in Ubuntu 8.10 i386

If you can reproduce the crash at will, may you read http://techbase.kde.org/Development/Tutorials/Debugging/How_to_create_useful_crash_reports and post a complete backtrace here? 
You need to install the "kdepim-dbg" package
Thanks :)

I guess I'm confronting the same issue here. It happens whenever I add a new filter and then try to apply it afterwards.

Fedora Core 10, KDE 4.2.1

Here's my backtrace (hope this will help you further, since I have debugging package installed)

--snip--
Application: KOrganizer (korganizer), signal SIGSEGV
[Current thread is 1 (Thread 0xb7f70770 (LWP 3382))]

Thread 1 (Thread 0xb7f70770 (LWP 3382)):
[KCrash Handler]
#6  0x005a1b3e in KOTodoModel::moveIfParentChanged (this=0xa02fec8, curNode=0xa417c10, todo=0xa41c8e8, addParentIfMissing=true)
    at /usr/src/debug/kdepim-4.2.1/korganizer/views/todoview/kotodomodel.cpp:378
#7  0x005a277a in KOTodoModel::reloadTodos (this=0xa02fec8) at /usr/src/debug/kdepim-4.2.1/korganizer/views/todoview/kotodomodel.cpp:254
#8  0x005aa6a0 in KOTodoView::updateView (this=0xa0070b0) at /usr/src/debug/kdepim-4.2.1/korganizer/views/todoview/kotodoview.cpp:354
#9  0x005c7b5e in CalendarView::updateView (this=0x91f6830, start=@0x92dc4cc, end=@0x92dc4cc, updateTodos=true) at /usr/src/debug/kdepim-4.2.1/korganizer/calendarview.cpp:815
#10 0x005c98fd in CalendarView::updateView (this=0x91f6830) at /usr/src/debug/kdepim-4.2.1/korganizer/calendarview.cpp:826
#11 0x005c7722 in CalendarView::filterActivated (this=0x91f6830, filterNo=4) at /usr/src/debug/kdepim-4.2.1/korganizer/calendarview.cpp:1738
#12 0x005d4dc6 in CalendarView::qt_metacall (this=0x91f6830, _c=QMetaObject::InvokeMetaMethod, _id=172, _a=0xbfeaa0cc)
    at /usr/src/debug/kdepim-4.2.1/i386-redhat-linux-gnu/korganizer/calendarview.moc:567
#13 0x01658750 in QMetaObject::activate (sender=0xa2fe2e0, from_signal_index=24, to_signal_index=24, argv=0xbfeaa0cc) at kernel/qobject.cpp:3031
#14 0x016594d2 in QMetaObject::activate (sender=0xa2fe2e0, m=0xfc0cb8, local_signal_index=1, argv=0xbfeaa0cc) at kernel/qobject.cpp:3101
#15 0x00d16823 in KSelectAction::triggered (this=0xa2fe2e0, _t1=4) at /usr/src/debug/kdelibs-4.2.1/i386-redhat-linux-gnu/kdeui/kselectaction.moc:165
#16 0x00d1829f in KSelectAction::actionTriggered (this=0xa2fe2e0, action=0xa65df48) at /usr/src/debug/kdelibs-4.2.1/kdeui/actions/kselectaction.cpp:312
#17 0x00d19564 in KSelectAction::qt_metacall (this=0xa2fe2e0, _c=QMetaObject::InvokeMetaMethod, _id=3, _a=0xbfeaa21c) at /usr/src/debug/kdelibs-4.2.1/i386-redhat-linux-gnu/kdeui/kselectaction.moc:104
#18 0x01658750 in QMetaObject::activate (sender=0xa2fe6b8, from_signal_index=4, to_signal_index=4, argv=0xbfeaa21c) at kernel/qobject.cpp:3031
#19 0x016594d2 in QMetaObject::activate (sender=0xa2fe6b8, m=0x1fe8c84, local_signal_index=0, argv=0xbfeaa21c) at kernel/qobject.cpp:3101
#20 0x01832833 in QActionGroup::triggered (this=0xa2fe6b8, _t1=0xa65df48) at .moc/release-shared/moc_qactiongroup.cpp:138
#21 0x018330e8 in QActionGroupPrivate::_q_actionTriggered (this=0xa2fe6c8) at kernel/qactiongroup.cpp:83
#22 0x018332fe in QActionGroup::qt_metacall (this=0xa2fe6b8, _c=QMetaObject::InvokeMetaMethod, _id=7, _a=0xbfeaa35c) at .moc/release-shared/moc_qactiongroup.cpp:94
#23 0x01658750 in QMetaObject::activate (sender=0xa65df48, from_signal_index=5, to_signal_index=6, argv=0xbfeaa35c) at kernel/qobject.cpp:3031
#24 0x01658b50 in QMetaObject::activate (sender=0xa65df48, m=0x1fe8c00, from_local_signal_index=1, to_local_signal_index=2, argv=0xbfeaa35c) at kernel/qobject.cpp:3121
#25 0x0182dcf1 in QAction::triggered (this=0xa65df48, _t1=true) at .moc/release-shared/moc_qaction.cpp:216
#26 0x0182e65f in QAction::activate (this=0xa65df48, event=QAction::Trigger) at kernel/qaction.cpp:1125
#27 0x01c0a7c4 in QMenuPrivate::activateAction (this=0xa2fe818, action=0xa65df48, action_e=QAction::Trigger, self=true) at widgets/qmenu.cpp:1002
#28 0x01c0b252 in QMenu::mouseReleaseEvent (this=0xa301710, e=0xbfeaaa38) at widgets/qmenu.cpp:2169
#29 0x00ec61b5 in KMenu::mouseReleaseEvent (this=0xa301710, e=0xbfeaaa38) at /usr/src/debug/kdelibs-4.2.1/kdeui/widgets/kmenu.cpp:454
#30 0x0188c052 in QWidget::event (this=0xa301710, event=0xbfeaaa38) at kernel/qwidget.cpp:7163
#31 0x01c0d549 in QMenu::event (this=0xa301710, e=0xbfeaaa38) at widgets/qmenu.cpp:2265
#32 0x01833fec in QApplicationPrivate::notify_helper (this=0x91ac550, receiver=0xa301710, e=0xbfeaaa38) at kernel/qapplication.cpp:3803
#33 0x0183c7e1 in QApplication::notify (this=0xbfeac3ac, receiver=0xa301710, e=0xbfeaaa38) at kernel/qapplication.cpp:3528
#34 0x00de70fd in KApplication::notify (this=0xbfeac3ac, receiver=0xa301710, event=0xbfeaaa38) at /usr/src/debug/kdelibs-4.2.1/kdeui/kernel/kapplication.cpp:307
#35 0x01643b21 in QCoreApplication::notifyInternal (this=0xbfeac3ac, receiver=0xa301710, event=0xbfeaaa38) at kernel/qcoreapplication.cpp:587
#36 0x0183ba6e in QCoreApplication::sendSpontaneousEvent (event=<value optimized out>, receiver=0xa301710) at ../../src/corelib/kernel/qcoreapplication.h:212
#37 QApplicationPrivate::sendMouseEvent (receiver=0xa301710, event=0xbfeaaa38, alienWidget=0x0, nativeWidget=0xa301710, buttonDown=0x1fffbd0, lastMouseReceiver=@0x1fffbd4)
    at kernel/qapplication.cpp:2793
#38 0x018a603e in QETWidget::translateMouseEvent (this=0xa301710, event=0xbfeab01c) at kernel/qapplication_x11.cpp:3980
#39 0x018a5145 in QApplication::x11ProcessEvent (this=0xbfeac3ac, event=0xbfeab01c) at kernel/qapplication_x11.cpp:3042
#40 0x018cd77a in x11EventSourceDispatch (s=0x91a3628, callback=0, user_data=0x0) at kernel/qguieventdispatcher_glib.cpp:142
#41 0x02f44238 in g_main_context_dispatch () from /lib/libglib-2.0.so.0
#42 0x02f478e3 in ?? () from /lib/libglib-2.0.so.0
#43 0x02f47aa1 in g_main_context_iteration () from /lib/libglib-2.0.so.0
#44 0x0166e158 in QEventDispatcherGlib::processEvents (this=0x9197910, flags={i = 4}) at kernel/qeventdispatcher_glib.cpp:319
#45 0x018cce75 in QGuiEventDispatcherGlib::processEvents (this=0x9197910, flags={i = 4}) at kernel/qguieventdispatcher_glib.cpp:198
#46 0x016421ea in QEventLoop::processEvents (this=0xbfeab27c, flags={i = 4}) at kernel/qeventloop.cpp:143
#47 0x016423aa in QEventLoop::exec (this=0xbfeab27c, flags={i = 0}) at kernel/qeventloop.cpp:194
#48 0x01c0d311 in QMenu::exec (this=0xa301710, p=@0xbfeab380, action=0x0) at widgets/qmenu.cpp:1938
#49 0x01c5487a in QToolButtonPrivate::popupTimerDone (this=0xa35b5c8) at widgets/qtoolbutton.cpp:905
#50 0x01c54f88 in QToolButtonPrivate::_q_buttonPressed (this=0xa35b5c8) at widgets/qtoolbutton.cpp:828
#51 0x01c55216 in QToolButton::qt_metacall (this=0xa35b588, _c=QMetaObject::InvokeMetaMethod, _id=4, _a=0xbfeab488) at .moc/release-shared/moc_qtoolbutton.cpp:107
#52 0x01658750 in QMetaObject::activate (sender=0xa35b588, from_signal_index=27, to_signal_index=27, argv=0x0) at kernel/qobject.cpp:3031
#53 0x016594d2 in QMetaObject::activate (sender=0xa35b588, m=0x1ff9244, local_signal_index=0, argv=0x0) at kernel/qobject.cpp:3101
#54 0x01e21067 in QAbstractButton::pressed (this=0xa35b588) at .moc/release-shared/moc_qabstractbutton.cpp:172
#55 0x01b73fc9 in QAbstractButtonPrivate::emitPressed (this=0xa35b5c8) at widgets/qabstractbutton.cpp:552
#56 0x01b741ae in QAbstractButton::mousePressEvent (this=0xa35b588, e=0xbfeabb18) at widgets/qabstractbutton.cpp:1087
#57 0x01c5594b in QToolButton::mousePressEvent (this=0xa35b588, e=0xbfeabb18) at widgets/qtoolbutton.cpp:662
#58 0x0188c039 in QWidget::event (this=0xa35b588, event=0xbfeabb18) at kernel/qwidget.cpp:7159
#59 0x01b742be in QAbstractButton::event (this=0xa35b588, e=0xbfeabb18) at widgets/qabstractbutton.cpp:1072
#60 0x01c52844 in QToolButton::event (this=0xa35b588, e=0xbfeabb18) at widgets/qtoolbutton.cpp:1103
#61 0x01833fec in QApplicationPrivate::notify_helper (this=0x91ac550, receiver=0xa35b588, e=0xbfeabb18) at kernel/qapplication.cpp:3803
#62 0x0183c7e1 in QApplication::notify (this=0xbfeac3ac, receiver=0xa35b588, e=0xbfeabb18) at kernel/qapplication.cpp:3528
#63 0x00de70fd in KApplication::notify (this=0xbfeac3ac, receiver=0xa35b588, event=0xbfeabb18) at /usr/src/debug/kdelibs-4.2.1/kdeui/kernel/kapplication.cpp:307
#64 0x01643b21 in QCoreApplication::notifyInternal (this=0xbfeac3ac, receiver=0xa35b588, event=0xbfeabb18) at kernel/qcoreapplication.cpp:587
#65 0x0183ba6e in QCoreApplication::sendSpontaneousEvent (event=<value optimized out>, receiver=0xa35b588) at ../../src/corelib/kernel/qcoreapplication.h:212
#66 QApplicationPrivate::sendMouseEvent (receiver=0xa35b588, event=0xbfeabb18, alienWidget=0xa35b588, nativeWidget=0x925a4b0, buttonDown=0x1fffbd0, lastMouseReceiver=@0x1fffbd4)
    at kernel/qapplication.cpp:2793
#67 0x018a5db6 in QETWidget::translateMouseEvent (this=0x925a4b0, event=0xbfeac0fc) at kernel/qapplication_x11.cpp:4046
#68 0x018a5145 in QApplication::x11ProcessEvent (this=0xbfeac3ac, event=0xbfeac0fc) at kernel/qapplication_x11.cpp:3042
#69 0x018cd77a in x11EventSourceDispatch (s=0x91a3628, callback=0, user_data=0x0) at kernel/qguieventdispatcher_glib.cpp:142
#70 0x02f44238 in g_main_context_dispatch () from /lib/libglib-2.0.so.0
#71 0x02f478e3 in ?? () from /lib/libglib-2.0.so.0
#72 0x02f47aa1 in g_main_context_iteration () from /lib/libglib-2.0.so.0
#73 0x0166e158 in QEventDispatcherGlib::processEvents (this=0x9197910, flags={i = 4}) at kernel/qeventdispatcher_glib.cpp:319
#74 0x018cce75 in QGuiEventDispatcherGlib::processEvents (this=0x9197910, flags={i = 4}) at kernel/qguieventdispatcher_glib.cpp:198
#75 0x016421ea in QEventLoop::processEvents (this=0xbfeac360, flags={i = 4}) at kernel/qeventloop.cpp:143
#76 0x016423aa in QEventLoop::exec (this=0xbfeac360, flags={i = 0}) at kernel/qeventloop.cpp:194
#77 0x01644a65 in QCoreApplication::exec () at kernel/qcoreapplication.cpp:845
#78 0x01833e67 in QApplication::exec () at kernel/qapplication.cpp:3331
#79 0x08050744 in main (argc=0, argv=0x0) at /usr/src/debug/kdepim-4.2.1/korganizer/main.cpp:59
--snip--

@Alex Back: your crash information trace is a different, could you post it in a new bug report ? Thanks!

@Darío Andrés
Done -> Bug #190251

*** Bug 190251 has been marked as a duplicate of this bug. ***

Haven't seen this crash in years.
Try with KDEPIM 4.6.