Version:            (using KDE KDE 3.2.1)
Installed from:    Gentoo Packages
Compiler:          gcc (GCC) 3.3.2 20031218 (Gentoo Linux 3.3.2-r5, propolice-3.3-7) 
OS:          Linux

It's happened a few times before, though not often. I haven't yet spotted a pattern. But it's always been after clicking the close button. Useally when I'm doing a clear up by closing all the windows I no longer need before logging out. Sorry I can't be more specific, hope the backtrace is some help:

Using host libthread_db library "/lib/libthread_db.so.1".
[Thread debugging using libthread_db enabled]
[New Thread 16384 (LWP 7183)]
0x410a47e8 in waitpid () from /lib/libpthread.so.0
#0  0x410a47e8 in waitpid () from /lib/libpthread.so.0
#1  0x407ee04c in __JCR_LIST__ () from /usr/kde/3.2/lib/libkdecore.so.4
#2  0x4072d0d3 in KCrash::defaultCrashHandler(int) ()
   from /usr/kde/3.2/lib/libkdecore.so.4
#3  0x410a34d3 in __pthread_sighandler () from /lib/libpthread.so.0
#4  <signal handler called>
#5  0x40dabd90 in QGList::findRef(void*, bool) ()
   from /usr/qt/3/lib/libqt-mt.so.3
#6  0x40dab9e5 in QGList::removeRef(void*) () from /usr/qt/3/lib/libqt-mt.so.3
#7  0x402253db in KDirListerCache::forgetDirs(KDirLister*, KURL const&, bool)
    () from /usr/kde/3.2/lib/libkio.so.4
#8  0x402252b3 in KDirListerCache::forgetDirs(KDirLister*) ()
   from /usr/kde/3.2/lib/libkio.so.4
#9  0x40229f7e in KDirLister::~KDirLister() ()
   from /usr/kde/3.2/lib/libkio.so.4
#10 0x44e264a5 in KonqSidebarDirTreeModule::~KonqSidebarDirTreeModule() ()
   from /usr/kde/3.2/lib/kde3/konq_sidebartree_dirtree.so
#11 0x44e199be in QPtrList<KonqSidebarTreeModule>::deleteItem(void*) ()
   from /usr/kde/3.2/lib/libkonq_sidebar_tree.so
#12 0x40dabd4e in QGList::clear() () from /usr/qt/3/lib/libqt-mt.so.3
#13 0x44e13e98 in KonqSidebarTree::clearTree() ()
   from /usr/kde/3.2/lib/libkonq_sidebar_tree.so
#14 0x44e13bdb in KonqSidebarTree::~KonqSidebarTree() ()
   from /usr/kde/3.2/lib/libkonq_sidebar_tree.so
#15 0x40b3845d in QWidget::~QWidget() () from /usr/qt/3/lib/libqt-mt.so.3
#16 0x405568ab in KDockWidget::~KDockWidget() ()
   from /usr/kde/3.2/lib/libkdeui.so.4
#17 0x4055a25d in KDockManager::~KDockManager() ()
   from /usr/kde/3.2/lib/libkdeui.so.4
#18 0x40566c9b in KDockArea::~KDockArea() ()
   from /usr/kde/3.2/lib/libkdeui.so.4
#19 0x40b3845d in QWidget::~QWidget() () from /usr/qt/3/lib/libqt-mt.so.3
#20 0x44d7e169 in Sidebar_Widget::~Sidebar_Widget() ()
   from /usr/kde/3.2/lib/kde3/konq_sidebar.so
#21 0x400330a2 in KParts::Part::~Part() () from /usr/kde/3.2/lib/libkparts.so.2
#22 0x400338da in KParts::ReadOnlyPart::~ReadOnlyPart() ()
   from /usr/kde/3.2/lib/libkparts.so.2
#23 0x44d73f8b in KonqSidebar::~KonqSidebar() ()
   from /usr/kde/3.2/lib/kde3/konq_sidebar.so
#24 0x416114bf in KonqView::~KonqView() ()
   from /usr/kde/3.2/lib/libkdeinit_konqueror.so
#25 0x4161c74d in KonqViewManager::clear() ()
   from /usr/kde/3.2/lib/libkdeinit_konqueror.so
#26 0x4165f42c in KonqMainWindow::stayPreloaded() ()
   from /usr/kde/3.2/lib/libkdeinit_konqueror.so
#27 0x4165cbcf in KonqMainWindow::queryExit() ()
   from /usr/kde/3.2/lib/libkdeinit_konqueror.so
#28 0x404c89a5 in KMainWindow::closeEvent(QCloseEvent*) ()
   from /usr/kde/3.2/lib/libkdeui.so.4
#29 0x4165c7b3 in KonqMainWindow::closeEvent(QCloseEvent*) ()
   from /usr/kde/3.2/lib/libkdeinit_konqueror.so
#30 0x40b3dd0f in QWidget::event(QEvent*) () from /usr/qt/3/lib/libqt-mt.so.3
#31 0x40bf7662 in QMainWindow::event(QEvent*) ()
   from /usr/qt/3/lib/libqt-mt.so.3
#32 0x4165f2e2 in KonqMainWindow::event(QEvent*) ()
   from /usr/kde/3.2/lib/libkdeinit_konqueror.so
#33 0x40aabfdf in QApplication::internalNotify(QObject*, QEvent*) ()
   from /usr/qt/3/lib/libqt-mt.so.3
#34 0x40aab3ab in QApplication::notify(QObject*, QEvent*) ()
   from /usr/qt/3/lib/libqt-mt.so.3
#35 0x40699e0c in KApplication::notify(QObject*, QEvent*) ()
   from /usr/kde/3.2/lib/libkdecore.so.4
#36 0x40b3d705 in QWidget::close(bool) () from /usr/qt/3/lib/libqt-mt.so.3
#37 0x40a457fa in QETWidget::translateCloseEvent(_XEvent const*) ()
   from /usr/qt/3/lib/libqt-mt.so.3
#38 0x40a3fc48 in QApplication::x11ClientMessage(QWidget*, _XEvent*, bool) ()
   from /usr/qt/3/lib/libqt-mt.so.3
#39 0x40a40bcb in QApplication::x11ProcessEvent(_XEvent*) ()
   from /usr/qt/3/lib/libqt-mt.so.3
#40 0x40a566e1 in QEventLoop::processEvents(unsigned) ()
   from /usr/qt/3/lib/libqt-mt.so.3
#41 0x40abf688 in QEventLoop::enterLoop() () from /usr/qt/3/lib/libqt-mt.so.3
#42 0x40abf538 in QEventLoop::exec() () from /usr/qt/3/lib/libqt-mt.so.3
#43 0x40aac231 in QApplication::exec() () from /usr/qt/3/lib/libqt-mt.so.3
#44 0x41604a7e in kdemain () from /usr/kde/3.2/lib/libkdeinit_konqueror.so
#45 0x408288c6 in kdeinitmain () from /usr/kde/3.2/lib/kde3/konqueror.so
#46 0x0804d1fe in launch(int, char const*, char const*, char const*, int, char const*, bool, char const*, bool, char const*) ()
#47 0x0804f7ff in handle_launcher_request(int) ()
#48 0x0804e67b in handle_requests(int) ()
#49 0x0804c520 in main ()

Regards,
David.