Version:           1.9.1 (using KDE 3.5.1, Kubuntu Package 4:3.5.1-0ubuntu7 dapper)
Compiler:          Target: i486-linux-gnu
OS:                Linux (i686) release 2.6.15-17-686

I selected all mails (~90) in my SPAM folder and pressed the "mark as spam" button in the toolbar. While KMail was doing the filtering, I select all mails in this folder (CTRL+A) and pressed Shift+del and confirmed the deletion. Then it crashed. This seems to be a race-condition or locking issue of some kind.

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 2012628672 (LWP 14101)]
[New Thread 1983490992 (LWP 14108)]
[New Thread 1991883696 (LWP 14107)]
[New Thread 2000276400 (LWP 14106)]
[New Thread 2008669104 (LWP 14105)]
[KCrash handler]
#6  0x00000000 in ?? ()
#7  0x4b7f34dc in KMail::MessageProperty::filtering ()
   from /usr/lib/libkmailprivate.so
#8  0x4b814bef in KMFilterMgr::beginFiltering ()
   from /usr/lib/libkmailprivate.so
#9  0x4b814cc9 in KMFilterMgr::process () from /usr/lib/libkmailprivate.so
#10 0x4b903a98 in KMFilterActionCommand::execute ()
   from /usr/lib/libkmailprivate.so
#11 0x4b8f8682 in KMCommand::slotPostTransfer ()
   from /usr/lib/libkmailprivate.so
#12 0x4b8fe0cb in KMCommand::qt_invoke () from /usr/lib/libkmailprivate.so
#13 0x4b8fe51b in KMFilterActionCommand::qt_invoke ()
   from /usr/lib/libkmailprivate.so
#14 0x49b48915 in QObject::activate_signal () from /usr/lib/libqt-mt.so.3
#15 0x4b8f75a4 in KMCommand::messagesTransfered ()
   from /usr/lib/libkmailprivate.so
#16 0x4b8fdc7f in KMCommand::transferSelectedMsgs ()
   from /usr/lib/libkmailprivate.so
#17 0x4b8fded6 in KMCommand::slotStart () from /usr/lib/libkmailprivate.so
#18 0x4b8fe0dd in KMCommand::qt_invoke () from /usr/lib/libkmailprivate.so
#19 0x4b8fe51b in KMFilterActionCommand::qt_invoke ()
   from /usr/lib/libkmailprivate.so
#20 0x49b48915 in QObject::activate_signal () from /usr/lib/libqt-mt.so.3
#21 0x49ed79be in QSignal::signal () from /usr/lib/libqt-mt.so.3
#22 0x49b66074 in QSignal::activate () from /usr/lib/libqt-mt.so.3
#23 0x49b6db64 in QSingleShotTimer::event () from /usr/lib/libqt-mt.so.3
#24 0x49aded02 in QApplication::internalNotify () from /usr/lib/libqt-mt.so.3
#25 0x49adeef4 in QApplication::notify () from /usr/lib/libqt-mt.so.3
#26 0x4a245d5d in KApplication::notify (this=0x7fd85c78, receiver=0x8279608, 
    event=0x7fd859d8) at kapplication.cpp:550
#27 0x49a6ffd3 in QApplication::sendEvent () from /usr/lib/libqt-mt.so.3
#28 0x49ad06ef in QEventLoop::activateTimers () from /usr/lib/libqt-mt.so.3
#29 0x49a83dbf in QEventLoop::processEvents () from /usr/lib/libqt-mt.so.3
#30 0x49af77fb in QEventLoop::enterLoop () from /usr/lib/libqt-mt.so.3
#31 0x49af771e in QEventLoop::exec () from /usr/lib/libqt-mt.so.3
#32 0x49add811 in QApplication::exec () from /usr/lib/libqt-mt.so.3
#33 0x0804a04b in ?? ()
#34 0x7fd85c78 in ?? ()
#35 0x7fd85e64 in ?? ()
#36 0x00000000 in ?? ()