Application that crashed: kontact
Version of the application: 4.3.0
KDE Version: 4.3.00 (KDE 4.3.0)
Qt Version: 4.5.2
Operating System: Linux 2.6.28-14-generic x86_64
Distribution: Ubuntu 9.04

What I was doing when the application crashed:
I added the  "Reply to All" button to the Main Toolbar (kmail_part), then Kontact crashed.  When I started Kontact back up, the button was there, and appears to work properly.  However, when I removed that button Kontact crashed again.  Readding the button did not cause it to crash again.  

 -- Backtrace:
Application: Kontact (kontact), signal: Aborted
[Current thread is 0 (LWP 3914)]

Thread 2 (Thread 0x7f4d510a5950 (LWP 3922)):
#0  __lll_lock_wait_private () at ../nptl/sysdeps/unix/sysv/linux/x86_64/lowlevellock.S:91
#1  0x00007f4d6d919025 in _L_lock_4783 () from /lib/libc.so.6
#2  0x00007f4d6d91526b in *__GI___libc_free (mem=0x7f4d6dc08a00) at malloc.c:3623
#3  0x00007f4d67c4fb2a in ?? () from /usr/lib/libglib-2.0.so.0
#4  0x00007f4d6e529a52 in ~QEventDispatcherGlib (this=0x3891a50) at kernel/qeventdispatcher_glib.cpp:289
#5  0x00007f4d6e415dfc in QThreadPrivate::finish (arg=<value optimized out>) at thread/qthread_unix.cpp:212
#6  0x00007f4d6e415d3d in QThreadPrivate::start (arg=0x18dc850) at /usr/include/pthread.h:533
#7  0x00007f4d6acef3ba in start_thread (arg=<value optimized out>) at pthread_create.c:297
#8  0x00007f4d6d980fcd in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:112
#9  0x0000000000000000 in ?? ()

Thread 1 (Thread 0x7f4d73f6e750 (LWP 3914)):
[KCrash Handler]
#5  0x00007f4d6d8cdfb5 in *__GI_raise (sig=<value optimized out>) at ../nptl/sysdeps/unix/sysv/linux/raise.c:64
#6  0x00007f4d6d8cfbc3 in *__GI_abort () at abort.c:88
#7  0x00007f4d6d90d228 in __libc_message (do_abort=2, fmt=0x7f4d6d9d7488 "*** glibc detected *** %s: %s: 0x%s ***\n") at ../sysdeps/unix/sysv/linux/libc_fatal.c:170
#8  0x00007f4d6d912cb8 in malloc_printerr (action=2, str=0x7f4d6d9d4a86 "free(): invalid pointer", ptr=<value optimized out>) at malloc.c:5994
#9  0x00007f4d6d915276 in *__GI___libc_free (mem=0x7f4d6d9cd8a0) at malloc.c:3625
#10 0x00007f4d6e50f5f5 in QObject::event (this=0x34e6920, e=0xf4a) at kernel/qobject.cpp:1094
#11 0x00007f4d6f0ce8ed in QWidget::event (this=0x34e6920, event=0x2d7baf0) at kernel/qwidget.cpp:7946
#12 0x00007f4d6f07df4d in QApplicationPrivate::notify_helper (this=0x18d4790, receiver=0x34e6920, e=0x2d7baf0) at kernel/qapplication.cpp:4056
#13 0x00007f4d6f08618a in QApplication::notify (this=0x7fff7bfbc1d0, receiver=0x34e6920, e=0x2d7baf0) at kernel/qapplication.cpp:4021
#14 0x00007f4d7048271b in KApplication::notify (this=0x7fff7bfbc1d0, receiver=0x34e6920, event=0x2d7baf0) at /build/buildd/kde4libs-4.3.0/kdeui/kernel/kapplication.cpp:302
#15 0x00007f4d6e4ff6ac in QCoreApplication::notifyInternal (this=0x7fff7bfbc1d0, receiver=0x34e6920, event=0x2d7baf0) at kernel/qcoreapplication.cpp:610
#16 0x00007f4d6e50031a in QCoreApplicationPrivate::sendPostedEvents (receiver=0x0, event_type=0, data=0x1897850) at ../../include/QtCore/../../src/corelib/kernel/qcoreapplication.h:213
#17 0x00007f4d6e528e03 in postEventSourceDispatch (s=<value optimized out>) at ../../include/QtCore/../../src/corelib/kernel/qcoreapplication.h:218
#18 0x00007f4d67c5020a in g_main_context_dispatch () from /usr/lib/libglib-2.0.so.0
#19 0x00007f4d67c538e0 in ?? () from /usr/lib/libglib-2.0.so.0
#20 0x00007f4d67c53a7c in g_main_context_iteration () from /usr/lib/libglib-2.0.so.0
#21 0x00007f4d6e528a8f in QEventDispatcherGlib::processEvents (this=0x18970a0, flags=<value optimized out>) at kernel/qeventdispatcher_glib.cpp:327
#22 0x00007f4d6f116bdf in QGuiEventDispatcherGlib::processEvents (this=0xf4a, flags=<value optimized out>) at kernel/qguieventdispatcher_glib.cpp:202
#23 0x00007f4d6e4fdf42 in QEventLoop::processEvents (this=<value optimized out>, flags={i = 2080096496}) at kernel/qeventloop.cpp:149
#24 0x00007f4d6e4fe314 in QEventLoop::exec (this=0x7fff7bfbc130, flags={i = 2080096576}) at kernel/qeventloop.cpp:201
#25 0x00007f4d6e5005e4 in QCoreApplication::exec () at kernel/qcoreapplication.cpp:888
#26 0x0000000000404945 in main (argc=1, argv=0x7fff7bfbc8b8) at /build/buildd/kdepim-4.3.0/kontact/src/main.cpp:218

Reported using DrKonqi