Version:           1.9.1 (using KDE 3.5.1, Kubuntu Package 4:3.5.1-0ubuntu5 dapper)
Compiler:          Target: i486-linux-gnu
OS:                Linux (i686) release 2.6.15-15-686

I somehow managed to crash KMail a way I never did before :-) I was trying to delete a bunch of spam mails while switching to a different folder. Sadly I can't reproduce it anymore.

Exact steps I did:

Pressed Ctl+A to select all mails in the spam folder
Pressed Shift+del to delete
Clicked on another folder
Activated konqueror to front
ca 2-4 seconds later KMail crashed in the background

Note, that when I switched to Konqueror KMail hadn't even processed/highlighted the selection of the CTl+A.

Backtrace follows:

(no debugging symbols found)
Using host libthread_db library "/lib/tls/i686/cmov/libthread_db.so.1".
(no debugging symbols found)
(no debugging symbols found)
(no debugging symbols found)
(no debugging symbols found)
(no debugging symbols found)
(no debugging symbols found)
[Thread debugging using libthread_db enabled]
[New Thread 899000000 (LWP 28969)]
[New Thread 893840304 (LWP 28973)]
[New Thread 894368688 (LWP 28972)]
[New Thread 894897072 (LWP 28971)]
[New Thread 895425456 (LWP 28970)]
[KCrash handler]
#6  0x08530969 in ?? ()
#7  0x37c7702c in KMail::MessageProperty::filtering ()
   from /usr/lib/libkmailprivate.so
#8  0x37c9873f in KMFilterMgr::beginFiltering ()
   from /usr/lib/libkmailprivate.so
#9  0x37c98819 in KMFilterMgr::process () from /usr/lib/libkmailprivate.so
#10 0x37d87608 in KMFilterActionCommand::execute ()
   from /usr/lib/libkmailprivate.so
#11 0x37d7c1f2 in KMCommand::slotPostTransfer ()
   from /usr/lib/libkmailprivate.so
#12 0x37d81c3b in KMCommand::qt_invoke () from /usr/lib/libkmailprivate.so
#13 0x37d8208b in KMFilterActionCommand::qt_invoke ()
   from /usr/lib/libkmailprivate.so
#14 0x370d026d in QObject::activate_signal () from /usr/lib/libqt-mt.so.3
#15 0x37d7b114 in KMCommand::messagesTransfered ()
   from /usr/lib/libkmailprivate.so
#16 0x37d817ef in KMCommand::transferSelectedMsgs ()
   from /usr/lib/libkmailprivate.so
#17 0x37d81a46 in KMCommand::slotStart () from /usr/lib/libkmailprivate.so
#18 0x37d81c4d in KMCommand::qt_invoke () from /usr/lib/libkmailprivate.so
#19 0x37d8208b in KMFilterActionCommand::qt_invoke ()
   from /usr/lib/libkmailprivate.so
#20 0x370d026d in QObject::activate_signal () from /usr/lib/libqt-mt.so.3
#21 0x3745f31e in QSignal::signal () from /usr/lib/libqt-mt.so.3
#22 0x370ed9cc in QSignal::activate () from /usr/lib/libqt-mt.so.3
#23 0x370f54bc in QSingleShotTimer::event () from /usr/lib/libqt-mt.so.3
#24 0x3706665a in QApplication::internalNotify () from /usr/lib/libqt-mt.so.3
#25 0x3706684c in QApplication::notify () from /usr/lib/libqt-mt.so.3
#26 0x3775ec7d in KApplication::notify (this=0x3f92bea8, receiver=0x8235230, 
    event=0x3f92bc08) at kapplication.cpp:550
#27 0x36ff7967 in QApplication::sendEvent () from /usr/lib/libqt-mt.so.3
#28 0x37058047 in QEventLoop::activateTimers () from /usr/lib/libqt-mt.so.3
#29 0x3700b753 in QEventLoop::processEvents () from /usr/lib/libqt-mt.so.3
#30 0x3707f153 in QEventLoop::enterLoop () from /usr/lib/libqt-mt.so.3
#31 0x3707f076 in QEventLoop::exec () from /usr/lib/libqt-mt.so.3
#32 0x37065169 in QApplication::exec () from /usr/lib/libqt-mt.so.3
#33 0x0804a04b in ?? ()
#34 0x3f92bea8 in ?? ()
#35 0x3f92c094 in ?? ()
#36 0x00000000 in ?? ()