Version:            (using KDE 3.5.8)
Installed from:    Debian testing/unstable Packages

I'm using KMail as Kontact Module.

What I did:
1. I created new rule.
2. Selected all messages in INBOX and pressed Ctrl+J (Apply filters)
There are about 450 messages in inbox.

Expected Behaviour:
Filters are applied to the selected messages.

Actual Behaviour:
KMail crashed.
The bug is reproducible for me. Below are two backtraces:

[KCrash handler]
#6  0x479df789 in KMFolder::find () from /usr/lib/libkmailprivate.so
#7  0x479a7c75 in KMHeaders::applyFiltersOnMsg ()
   from /usr/lib/libkmailprivate.so
#8  0x47b468ba in KMMainWidget::slotApplyFilters ()
   from /usr/lib/libkmailprivate.so
#9  0x47b60cdc in KMMainWidget::qt_invoke () from /usr/lib/libkmailprivate.so
#10 0x471eb0cc in QObject::activate_signal () from /usr/lib/libqt-mt.so.3
#11 0x471ebbb1 in QObject::activate_signal () from /usr/lib/libqt-mt.so.3
#12 0x460375e9 in KAction::activated () from /usr/lib/libkdeui.so.4
#13 0x460766c2 in KAction::slotActivated () from /usr/lib/libkdeui.so.4
#14 0x461656f1 in KAction::qt_invoke () from /usr/lib/libkdeui.so.4
#15 0x471eb0cc in QObject::activate_signal () from /usr/lib/libqt-mt.so.3
#16 0x471ebbb1 in QObject::activate_signal () from /usr/lib/libqt-mt.so.3
#17 0x45d67b69 in KAccelPrivate::menuItemActivated ()
   from /usr/lib/libkdecore.so.4
#18 0x45db6db7 in KAccelPrivate::emitActivatedSignal ()
   from /usr/lib/libkdecore.so.4
#19 0x45e45568 in KAccelPrivate::eventFilter () from /usr/lib/libkdecore.so.4
#20 0x471e8e5a in QObject::activate_filters () from /usr/lib/libqt-mt.so.3
#21 0x471e8ed8 in QObject::event () from /usr/lib/libqt-mt.so.3
#22 0x47221944 in QWidget::event () from /usr/lib/libqt-mt.so.3
#23 0x4717fa66 in QApplication::internalNotify () from /usr/lib/libqt-mt.so.3
#24 0x47181a30 in QApplication::notify () from /usr/lib/libqt-mt.so.3
#25 0x45e4fcf2 in KApplication::notify () from /usr/lib/libkdecore.so.4
#26 0x45dc2df0 in KAccelEventHandler::x11Event ()
   from /usr/lib/libkdecore.so.4
#27 0x45e4e16a in KApplication::x11EventFilter ()
   from /usr/lib/libkdecore.so.4
#28 0x470fcab0 in ?? () from /usr/lib/libqt-mt.so.3
#29 0xbfd8fdb8 in ?? ()
#30 0xbfd8fc18 in ?? ()
#31 0x00000001 in ?? ()
#32 0x006be5d0 in ?? ()
#33 0xbfd8fdb8 in ?? ()
#34 0x476be5d0 in ?? () from /usr/lib/libqt-mt.so.3
#35 0xbfd8fb58 in ?? ()
#36 0x4710cddc in QApplication::x11ProcessEvent () from /usr/lib/libqt-mt.so.3

[KCrash handler]
#6  0x479df792 in KMFolder::find () from /usr/lib/libkmailprivate.so
#7  0x479a7c75 in KMHeaders::applyFiltersOnMsg ()
   from /usr/lib/libkmailprivate.so
#8  0x47b468ba in KMMainWidget::slotApplyFilters ()
   from /usr/lib/libkmailprivate.so
#9  0x47b60cdc in KMMainWidget::qt_invoke () from /usr/lib/libkmailprivate.so
#10 0x471eb0cc in QObject::activate_signal () from /usr/lib/libqt-mt.so.3
#11 0x471ebbb1 in QObject::activate_signal () from /usr/lib/libqt-mt.so.3
#12 0x460375e9 in KAction::activated () from /usr/lib/libkdeui.so.4
#13 0x460766c2 in KAction::slotActivated () from /usr/lib/libkdeui.so.4
#14 0x461656f1 in KAction::qt_invoke () from /usr/lib/libkdeui.so.4
#15 0x471eb0cc in QObject::activate_signal () from /usr/lib/libqt-mt.so.3
#16 0x471ebbb1 in QObject::activate_signal () from /usr/lib/libqt-mt.so.3
#17 0x45d67b69 in KAccelPrivate::menuItemActivated ()
   from /usr/lib/libkdecore.so.4
#18 0x45db6db7 in KAccelPrivate::emitActivatedSignal ()
   from /usr/lib/libkdecore.so.4
#19 0x45e45568 in KAccelPrivate::eventFilter () from /usr/lib/libkdecore.so.4
#20 0x471e8e5a in QObject::activate_filters () from /usr/lib/libqt-mt.so.3
#21 0x471e8ed8 in QObject::event () from /usr/lib/libqt-mt.so.3
#22 0x47221944 in QWidget::event () from /usr/lib/libqt-mt.so.3
#23 0x472f778a in QMainWindow::event () from /usr/lib/libqt-mt.so.3
#24 0x4717fa66 in QApplication::internalNotify () from /usr/lib/libqt-mt.so.3
#25 0x47181a30 in QApplication::notify () from /usr/lib/libqt-mt.so.3
#26 0x45e4fcf2 in KApplication::notify () from /usr/lib/libkdecore.so.4
#27 0x45dc2df0 in KAccelEventHandler::x11Event ()
   from /usr/lib/libkdecore.so.4
#28 0x45e4e16a in KApplication::x11EventFilter ()
   from /usr/lib/libkdecore.so.4
#29 0x470fcab0 in ?? () from /usr/lib/libqt-mt.so.3
#30 0xbfcff528 in ?? ()
#31 0xbfcff388 in ?? ()
#32 0x00000001 in ?? ()
#33 0x006be5d0 in ?? ()
#34 0xbfcff528 in ?? ()
#35 0x476be5d0 in ?? () from /usr/lib/libqt-mt.so.3
#36 0xbfcff2c8 in ?? ()
#37 0x4710cddc in QApplication::x11ProcessEvent () from /usr/lib/libqt-mt.so.3