Version:           1.9.5 (using KDE 3.5.5, Debian Package 4:3.5.5a.dfsg.1-2 (testing/unstable))
Compiler:          Target: i486-linux-gnu
OS:                Linux (i686) release 2.6.17

Sometimes when I remove a mail kmail stops reacting and uses 100% CPU. IIRC it only happens when I use the option "Folder -> Thread Messages" and remove a mail that is top of a thread.
This time I attached gdb to kontact after it started eating my CPU. Here is the backtrace:
-------------
#0  0xa715e207 in QListViewItem::listView () from /usr/lib/libqt-mt.so.3
#1  0xa68c14f4 in KListViewItem::insertItem () from /usr/lib/libkdeui.so.4
#2  0xa54ef94f in KMHeaders::msgRemoved () from /usr/lib/libkmailprivate.so
#3  0xa55f2c29 in KMHeaders::qt_invoke () from /usr/lib/libkmailprivate.so
#4  0xa706ccb3 in QObject::activate_signal () from /usr/lib/libqt-mt.so.3
#5  0xa53fb340 in KMFolder::msgRemoved () from /usr/lib/libkmailprivate.so
#6  0xa54e499d in KMFolder::qt_emit () from /usr/lib/libkmailprivate.so
#7  0xa706cc87 in QObject::activate_signal () from /usr/lib/libqt-mt.so.3
#8  0xa53fa450 in FolderStorage::msgRemoved () from /usr/lib/libkmailprivate.so
#9  0xa555c33b in FolderStorage::take () from /usr/lib/libkmailprivate.so
#10 0xa5626eac in KMFolderMaildir::take () from /usr/lib/libkmailprivate.so
#11 0xa53b72ae in KMFolder::take () from /usr/lib/libkmailprivate.so
#12 0xa56182e6 in KMFolderMaildir::addMsgInternal () from /usr/lib/libkmailprivate.so
#13 0xa5618c25 in KMFolderMaildir::addMsg () from /usr/lib/libkmailprivate.so
#14 0xa53e0c23 in FolderStorage::moveMsg () from /usr/lib/libkmailprivate.so
#15 0xa53b73e5 in KMFolder::moveMsg () from /usr/lib/libkmailprivate.so
#16 0xa54832ef in KMMoveCommand::execute () from /usr/lib/libkmailprivate.so
#17 0xa53fe789 in KMCommand::slotPostTransfer () from /usr/lib/libkmailprivate.so
#18 0xa5484c26 in KMCommand::qt_invoke () from /usr/lib/libkmailprivate.so
#19 0xa5484ffb in KMMenuCommand::qt_invoke () from /usr/lib/libkmailprivate.so
#20 0xa5485093 in KMMoveCommand::qt_invoke () from /usr/lib/libkmailprivate.so
#21 0xa548510b in KMDeleteMsgCommand::qt_invoke () from /usr/lib/libkmailprivate.so
#22 0xa706ccb3 in QObject::activate_signal () from /usr/lib/libqt-mt.so.3
#23 0xa53fe8ae in KMCommand::messagesTransfered () from /usr/lib/libkmailprivate.so
#24 0xa54e53d4 in KMCommand::transferSelectedMsgs () from /usr/lib/libkmailprivate.so
#25 0xa54e5537 in KMCommand::slotStart () from /usr/lib/libkmailprivate.so
#26 0xa5484c38 in KMCommand::qt_invoke () from /usr/lib/libkmailprivate.so
#27 0xa5484ffb in KMMenuCommand::qt_invoke () from /usr/lib/libkmailprivate.so
#28 0xa5485093 in KMMoveCommand::qt_invoke () from /usr/lib/libkmailprivate.so
#29 0xa548510b in KMDeleteMsgCommand::qt_invoke () from /usr/lib/libkmailprivate.so
#30 0xa706ccb3 in QObject::activate_signal () from /usr/lib/libqt-mt.so.3
#31 0xa73f5cef in QSignal::signal () from /usr/lib/libqt-mt.so.3
#32 0xa708c836 in QSignal::activate () from /usr/lib/libqt-mt.so.3
#33 0xa7094208 in QSingleShotTimer::event () from /usr/lib/libqt-mt.so.3
#34 0xa7004bd6 in QApplication::internalNotify () from /usr/lib/libqt-mt.so.3
#35 0xa70069f3 in QApplication::notify () from /usr/lib/libqt-mt.so.3
#36 0xa772ce0e in KApplication::notify () from /usr/lib/libkdecore.so.4
#37 0xa6f983d1 in QApplication::sendEvent () from /usr/lib/libqt-mt.so.3
#38 0xa6ff75d3 in QEventLoop::activateTimers () from /usr/lib/libqt-mt.so.3
#39 0xa6fac71f in QEventLoop::processEvents () from /usr/lib/libqt-mt.so.3
#40 0xa701f129 in QEventLoop::enterLoop () from /usr/lib/libqt-mt.so.3
#41 0xa701ef4a in QEventLoop::exec () from /usr/lib/libqt-mt.so.3
#42 0xa700676f in QApplication::exec () from /usr/lib/libqt-mt.so.3
#43 0x0805ac3f in ?? ()