Version:           4.0 (using KDE 3.1.0 (KDE 3.1 RC4))
Installed from:    compiled sources
Compiler:          gcc version 3.2.1 [FreeBSD] 20021009 (prerelease)
OS:          FreeBSD (i386) release 5.0-CURRENT

I can repeat this every time.

Open Konqueror, open another tab (how it's opened doesn't matter), close the second tab, and Konqueror exits.

If I open Konqueror, and open several more tabs, then close them one by one, when I am down to two, closing the next one makes Konqueror exit.

backtrace:
#0  0x28b4f2f5 in QApplication::notify(QObject*, QEvent*) ()
   from /usr/X11R6/qt-cvs/lib/libqt-mt.so.3
#1  0x287ceb9d in KApplication::notify(QObject*, QEvent*) (this=0xbfbff7d8,
    receiver=0x8087a00, event=0xbfbff240) at /cvs/kdelibs/kdecore/kapplication.cpp:437
#2  0x28af67ce in QETWidget::translateMouseEvent(_XEvent const*) ()
   from /usr/X11R6/qt-cvs/lib/libqt-mt.so.3
#3  0x28af438f in QApplication::x11ProcessEvent(_XEvent*) ()
   from /usr/X11R6/qt-cvs/lib/libqt-mt.so.3
#4  0x28b089ce in QEventLoop::processEvents(unsigned) ()
   from /usr/X11R6/qt-cvs/lib/libqt-mt.so.3
#5  0x28b6381f in QEventLoop::enterLoop() () from /usr/X11R6/qt-cvs/lib/libqt-mt.so.3
#6  0x28b636dc in QEventLoop::exec() () from /usr/X11R6/qt-cvs/lib/libqt-mt.so.3
#7  0x28b4fbb0 in QApplication::exec() () from /usr/X11R6/qt-cvs/lib/libqt-mt.so.3
#8  0x280b92d8 in main (argc=1, argv=0xbfbff90c)
    at /cvs/kdebase/konqueror/konq_main.cc:130
#9  0x080485cc in _start ()


Konsole debug info in case it helps:
konqueror: ---------------- KonqViewManager::removeTab( 0x81fb574 ) --------------
konqueror: 2Views
konqueror: 0x81f3600
konqueror: 0x843fc00
konqueror: currentView=0x843fc00
konqueror: docContainer=0x81f3378
konqueror: KonqFrameTabs [KonqFrameTabs pointer (0x81f3300) to unnamed widget, geometry=1281x859+0+130] visible=1 activeChild=0x81fb574
konqueror:   KonqFrame [KonqFrame pointer (0x8145c00) to widget KonqFrame, geometry=98x1+1+1] visible=0 containing view 0x81f3600 visible=0 and part 0x82fb100 whose widget is a KHTMLView
konqueror:   KonqFrame [KonqFrame pointer (0x81fb500) to widget KonqFrame, geometry=1279x827+1+1] visible=1 containing view 0x843fc00 visible=1 and part 0x843fd00 whose widget is a KHTMLView
kparts: 0x810e300 emitting activePartChanged 0x0
konqueror: KonqMainWindow::slotPartActivated 0x0
konqueror: New current view 0x0
konqueror: No part activated - returning
kparts: MainWindow::createGUI, part=0x0
kparts: deactivating GUI for 0x843fd00 KHTMLPart
kparts: 0x810e300 emitting activePartChanged 0x82fb100
konqueror: Part is already active!
konqueror: KonqMainWindow::removeChildView childView 0x843fc00
konqueror: 2Views
konqueror: 0x81f3600
konqueror: 0x843fc00
konqueror: Searching map
konqueror: Verifying search results
konqueror: Removing view 0x843fc00
konqueror: View 0x843fc00 removed from map
konqueror: KonqMainWindow::viewCountChanged
konqueror: 1Views
konqueror: 0x81f3600
konqueror: KonqView::~KonqView : part = 0x843fd00
khtml (jscript): Window::clear 0x8423d80
khtml (jscript): Window::~Window this=0x8423d80 part=0x843fd00
kparts: Part::~Part 0x843fd00
konqueror: KonqViewManager::removePart ( 0x843fd00 )
konqueror: Calling KParts::PartManager::removePart 0x843fd00
konqueror: KonqViewManager::removePart ( 0x843fd00 ) done
kparts: deleting widget [KHTMLView pointer (0x843ff00) to widget view widget, geometry=1279x807+0+0] view widget
konqueror: KonqView::~KonqView 0x843fc00 done
konqueror: parentContainer=0x80d7cb4
konqueror: otherFrame=0x8145c74
konqueror: Part is already active!
konqueror: 1Views
konqueror: 0x81f3600
konqueror: currentView=0x0
konqueror: docContainer=0x8145c74
konqueror: KonqFrame [KonqFrame pointer (0x8145c00) to widget KonqFrame, geometry=1279x827+1+1] visible=1 containing view 0x81f3600 visible=1 and part 0x82fb100 whose widget is a KHTMLView
konqueror: ------------- KonqViewManager::removeTab done --------------

Program received signal SIGBUS, Bus error.