I was selecting a new email in kmail in order to read it when kmail crashed.
I decided to file this under nepomuk because the kmail backtrace doesn't seem to contain much information.

I couldn't reproduce the crash.
The KDE version is 4.8.1.

Application: KMail (kmail), signal: Aborted
[Current thread is 1 (Thread 0x7fb041075780 (LWP 13161))]

Thread 4 (Thread 0x7fb022dde700 (LWP 13165)):
#0  0x00007fb03c1bee6c in pthread_cond_wait@@GLIBC_2.3.2 () from /lib64/libpthread.so.0
#1  0x00007fb032877c12 in ?? () from /usr/lib64/libQtWebKit.so.4
#2  0x00007fb032877c49 in ?? () from /usr/lib64/libQtWebKit.so.4
#3  0x00007fb03c1baf05 in start_thread () from /lib64/libpthread.so.0
#4  0x00007fb03e3b163d in clone () from /lib64/libc.so.6

Thread 3 (Thread 0x7fb0224dd700 (LWP 13166)):
#0  0x00007fb03e3a8523 in poll () from /lib64/libc.so.6
#1  0x00007fb036591a98 in ?? () from /usr/lib64/libglib-2.0.so.0
#2  0x00007fb036591f59 in g_main_context_iteration () from /usr/lib64/libglib-2.0.so.0
#3  0x00007fb03fa0e956 in QEventDispatcherGlib::processEvents (this=0x7fb01c0008c0, flags=<optimized out>) at kernel/qeventdispatcher_glib.cpp:426
#4  0x00007fb03f9de682 in QEventLoop::processEvents (this=<optimized out>, flags=...) at kernel/qeventloop.cpp:149
#5  0x00007fb03f9de8d7 in QEventLoop::exec (this=0x7fb0224dce00, flags=...) at kernel/qeventloop.cpp:204
#6  0x00007fb03f8df537 in QThread::exec (this=<optimized out>) at thread/qthread.cpp:501
#7  0x00007fb03f8e255b in QThreadPrivate::start (arg=0xb93b30) at thread/qthread_unix.cpp:298
#8  0x00007fb03c1baf05 in start_thread () from /lib64/libpthread.so.0
#9  0x00007fb03e3b163d in clone () from /lib64/libc.so.6

Thread 2 (Thread 0x7fafdb53e700 (LWP 18410)):
[KCrash Handler]
#10 0x00007fb03e34cf60 in malloc_consolidate () from /lib64/libc.so.6
#11 0x00007fb03e34e123 in _int_malloc () from /lib64/libc.so.6
#12 0x00007fb03e350d6e in malloc () from /lib64/libc.so.6
#13 0x00007fb03f8e4558 in QByteArray::realloc (this=0x7fafd0002868, alloc=65504) at tools/qbytearray.cpp:1452
#14 0x00007fb03f8e4679 in QByteArray::resize (this=0x7fafd0002868, size=32768) at tools/qbytearray.cpp:1420
#15 0x00007fb03d3d7986 in reserve (bytes=2, this=0x7fafd00027c0) at ../../src/corelib/tools/qringbuffer_p.h:158
#16 QAbstractSocket::writeData (this=<optimized out>, data=0x7fafdb53d62c "\021", size=2) at socket/qabstractsocket.cpp:2315
#17 0x00007fb03f97282f in QIODevice::write (this=0x7fafd00046b0, data=0x7fafdb53d62c "\021", maxSize=<optimized out>) at io/qiodevice.cpp:1370
#18 0x00007fb02ffa3b25 in ?? () from /usr/lib64/libsopranoclient.so.1
#19 0x00007fb02ffaacba in ?? () from /usr/lib64/libsopranoclient.so.1
#20 0x00007fb02ffac803 in ?? () from /usr/lib64/libsopranoclient.so.1
#21 0x00007fb03719664c in Nepomuk::MainModel::executeQuery (this=0xb2c660, query=..., language=Soprano::Query::QueryLanguageSparql, userQueryLanguage=...) at /usr/src/debug/kdelibs-4.8.1/nepomuk/core/nepomukmainmodel.cpp:212
#22 0x00007fb037182eb3 in Nepomuk::ResourceData::load (this=0x7fafd00057b0) at /usr/src/debug/kdelibs-4.8.1/nepomuk/core/resourcedata.cpp:417
#23 0x00007fb0371837bc in Nepomuk::ResourceData::property (this=0x7fafd00057b0, uri=...) at /usr/src/debug/kdelibs-4.8.1/nepomuk/core/resourcedata.cpp:265
#24 0x00007fb0371997e3 in Nepomuk::Resource::property (this=0x7fafdb53dd00, uri=...) at /usr/src/debug/kdelibs-4.8.1/nepomuk/core/resource.cpp:301
#25 0x00007fb03a5000ca in ?? () from /usr/lib64/libmessagecore.so.4
#26 0x00007fb03f8d5d12 in QThreadPoolThread::run (this=0x11ef1c0) at concurrent/qthreadpool.cpp:107
#27 0x00007fb03f8e255b in QThreadPrivate::start (arg=0x11ef1c0) at thread/qthread_unix.cpp:298
#28 0x00007fb03c1baf05 in start_thread () from /lib64/libpthread.so.0
#29 0x00007fb03e3b163d in clone () from /lib64/libc.so.6

Thread 1 (Thread 0x7fb041075780 (LWP 13161)):
[KCrash Handler]
#6  0x00007fb03e30cd95 in raise () from /lib64/libc.so.6
#7  0x00007fb03e30e2ab in abort () from /lib64/libc.so.6
#8  0x00007fb03e346ece in __libc_message () from /lib64/libc.so.6
#9  0x00007fb03e34cc06 in malloc_printerr () from /lib64/libc.so.6
#10 0x00007fb03c42a54e in qMetaTypeDeleteHelper<QDBusMessage> (t=0x1476170) at ../../src/corelib/kernel/qmetatype.h:134
#11 0x00007fb03f9efd79 in QMetaCallEvent::~QMetaCallEvent (this=0x1475a40, __in_chrg=<optimized out>) at kernel/qobject.cpp:509
#12 0x00007fb03f9efdd9 in QMetaCallEvent::~QMetaCallEvent (this=0x1475a40, __in_chrg=<optimized out>) at kernel/qobject.cpp:518
#13 0x00007fb03f9e3143 in QCoreApplicationPrivate::sendPostedEvents (receiver=0x0, event_type=0, data=0x60b720) at kernel/qcoreapplication.cpp:1516
#14 0x00007fb03fa0e4c3 in sendPostedEvents () at kernel/qcoreapplication.h:236
#15 postEventSourceDispatch (s=<optimized out>) at kernel/qeventdispatcher_glib.cpp:279
#16 0x00007fb03659158d in g_main_context_dispatch () from /usr/lib64/libglib-2.0.so.0
#17 0x00007fb036591d88 in ?? () from /usr/lib64/libglib-2.0.so.0
#18 0x00007fb036591f59 in g_main_context_iteration () from /usr/lib64/libglib-2.0.so.0
#19 0x00007fb03fa0e8ef in QEventDispatcherGlib::processEvents (this=0x60b000, flags=<optimized out>) at kernel/qeventdispatcher_glib.cpp:424
#20 0x00007fb03ee082de in QGuiEventDispatcherGlib::processEvents (this=<optimized out>, flags=<optimized out>) at kernel/qguieventdispatcher_glib.cpp:204
#21 0x00007fb03f9de682 in QEventLoop::processEvents (this=<optimized out>, flags=...) at kernel/qeventloop.cpp:149
#22 0x00007fb03f9de8d7 in QEventLoop::exec (this=0x7fffda4c2410, flags=...) at kernel/qeventloop.cpp:204
#23 0x00007fb03f9e3435 in QCoreApplication::exec () at kernel/qcoreapplication.cpp:1148
#24 0x0000000000402df3 in main (argc=<optimized out>, argv=<optimized out>) at /usr/src/debug/kdepim-4.8.1/kmail/main.cpp:145