Version:            (using KDE KDE 3.5.7)
Installed from:    Mandriva RPMs
OS:                Linux

It is easy to reproduce:

1. Take any message and delete it (move to the trashcan).
2. Empty all Trash folder.
3. Click Undo.

I don't think backtrace is important, but here it is:

[KCrash handler]
#6  0xb7ac032b in FolderStorage::moveMsg () from /usr/lib/libkmailprivate.so
#7  0xb7a92b15 in KMFolder::moveMsg () from /usr/lib/libkmailprivate.so
#8  0xb7d26c34 in KMail::UndoStack::undo () from /usr/lib/libkmailprivate.so
#9  0xb7d26d82 in KMHeaders::undo () from /usr/lib/libkmailprivate.so
#10 0xb7c992cb in KMMainWidget::slotUndo () from /usr/lib/libkmailprivate.so
#11 0xb7db21cd in KMMainWidget::qt_invoke () from /usr/lib/libkmailprivate.so
#12 0xb5e5df5d in QObject::activate_signal ()
   from /usr/lib/qt3/lib/libqt-mt.so.3
#13 0xb5e5eadd in QObject::activate_signal ()
   from /usr/lib/qt3/lib/libqt-mt.so.3
#14 0xb672a4e7 in KAction::activated () from /usr/lib/libkdeui.so.4
#15 0xb6761372 in KAction::slotActivated () from /usr/lib/libkdeui.so.4
#16 0xb686156a in KAction::slotPopupActivated () from /usr/lib/libkdeui.so.4
#17 0xb6861865 in KAction::qt_invoke () from /usr/lib/libkdeui.so.4
#18 0xb5e5df5d in QObject::activate_signal ()
   from /usr/lib/qt3/lib/libqt-mt.so.3
#19 0xb616b33e in QSignal::signal () from /usr/lib/qt3/lib/libqt-mt.so.3
#20 0xb5e798b7 in QSignal::activate () from /usr/lib/qt3/lib/libqt-mt.so.3
#21 0xb5f6070a in QPopupMenu::mouseReleaseEvent ()
   from /usr/lib/qt3/lib/libqt-mt.so.3
#22 0xb67342b4 in KPopupMenu::mouseReleaseEvent () from /usr/lib/libkdeui.so.4
#23 0xb5e96564 in QWidget::event () from /usr/lib/qt3/lib/libqt-mt.so.3
#24 0xb5e00cea in QApplication::internalNotify ()
   from /usr/lib/qt3/lib/libqt-mt.so.3
#25 0xb5e01cca in QApplication::notify () from /usr/lib/qt3/lib/libqt-mt.so.3
#26 0xb64fbdba in KApplication::notify () from /usr/lib/libkdecore.so.4
#27 0xb5da29f8 in QETWidget::translateMouseEvent ()
   from /usr/lib/qt3/lib/libqt-mt.so.3
#28 0xb5da1156 in QApplication::x11ProcessEvent ()
   from /usr/lib/qt3/lib/libqt-mt.so.3
#29 0xb5db12ad in QEventLoop::processEvents ()
   from /usr/lib/qt3/lib/libqt-mt.so.3
#30 0xb5e169e0 in QEventLoop::enterLoop () from /usr/lib/qt3/lib/libqt-mt.so.3
#31 0xb5e16876 in QEventLoop::exec () from /usr/lib/qt3/lib/libqt-mt.so.3
#32 0xb5e0090f in QApplication::exec () from /usr/lib/qt3/lib/libqt-mt.so.3
#33 0x0804a46e in QObject::checkConnectArgs ()
#34 0xb52fff90 in __libc_start_main () from /lib/i686/libc.so.6
#35 0x0804a1c1 in QObject::checkConnectArgs ()