I get a consistent crash (seg fault) every time I quit KTorrent 4.1. I am running Kubuntu 10.10 with KDE 4.6.2a. I compiled KT myself with no problems at all. It runs great except for the seg fault. I am also going to submit this as a bug report on bugs.kde.org.

Here is the backtrace:

Application: KTorrent (ktorrent), signal: Segmentation fault
[Current thread is 1 (Thread 0xb77ce710 (LWP 23380))]

Thread 3 (Thread 0xaecc9b70 (LWP 23390)):
#0 0x00bd33f4 in __pthread_mutex_lock (mutex=0x88d377c) at pthread_mutex_lock.c:62
#1 0x036acf66 in pthread_mutex_lock (mutex=0x88d377c) at forward.c:182
#2 0x07df9ea3 in g_main_context_prepare () from /lib/libglib-2.0.so.0
#3 0x07dfa279 in ?? () from /lib/libglib-2.0.so.0
#4 0x07dfa848 in g_main_context_iteration () from /lib/libglib-2.0.so.0
#5 0x01acb59f in QEventDispatcherGlib::processEvents (this=0x86b7670, flags=...) at kernel/qeventdispatcher_glib.cpp:417
#6 0x01a9b609 in QEventLoop::processEvents (this=0xaecc9290, flags=) at kernel/qeventloop.cpp:149
#7 0x01a9ba8a in QEventLoop::exec (this=0xaecc9290, flags=...) at kernel/qeventloop.cpp:201
#8 0x01997b7e in QThread::exec (this=0x88c8c48) at thread/qthread.cpp:490
#9 0x01a7a35b in QInotifyFileSystemWatcherEngine::run (this=0x88c8c48) at io/qfilesystemwatcher_inotify.cpp:248
#10 0x0199adf9 in QThreadPrivate::start (arg=0x88c8c48) at thread/qthread_unix.cpp:266
#11 0x00bd0cc9 in start_thread (arg=0xaecc9b70) at pthread_create.c:304
#12 0x0369f69e in clone () at ../sysdeps/unix/sysv/linux/i386/clone.S:130

Thread 2 (Thread 0xae4c8b70 (LWP 23391)):
#0 0x00bf1416 in __kernel_vsyscall ()
#1 0x00bd54dc in pthread_cond_wait@@GLIBC_2.3.2 () at ../nptl/sysdeps/unix/sysv/linux/i386/i686/../i486/pthread_cond_wait.S:169
#2 0x036acd9d in __pthread_cond_wait (cond=0x88d0f88, mutex=0x88d0f70) at forward.c:139
#3 0x0199b9c7 in wait (this=0x88d064c, mutex=0x88d0648, time=4294967295) at thread/qwaitcondition_unix.cpp:88
#4 QWaitCondition::wait (this=0x88d064c, mutex=0x88d0648, time=4294967295) at thread/qwaitcondition_unix.cpp:160
#5 0x0032ea12 in net::ReverseResolverThread::run (this=0x88d0640) at /home/rj45/Downloads/libktorrent-1.1.0/src/net/reverseresolver.cpp:123
#6 0x0199adf9 in QThreadPrivate::start (arg=0x88d0640) at thread/qthread_unix.cpp:266
#7 0x00bd0cc9 in start_thread (arg=0xae4c8b70) at pthread_create.c:304
#8 0x0369f69e in clone () at ../sysdeps/unix/sysv/linux/i386/clone.S:130

Thread 1 (Thread 0xb77ce710 (LWP 23380)):
[KCrash Handler]
#7 operator++ (this=0x811faf0, sock=0x86a12d8) at /usr/include/c++/4.4/bits/stl_list.h:223
#8 __distance<std::_List_const_iterator<net::BufferedSocket*> > (this=0x811faf0, sock=0x86a12d8) at /usr/include/c++/4.4/bits/stl_iterator_base_funcs.h:79
#9 distance<std::_List_const_iterator<net::BufferedSocket*> > (this=0x811faf0, sock=0x86a12d8) at /usr/include/c++/4.4/bits/stl_iterator_base_funcs.h:114
#10 size (this=0x811faf0, sock=0x86a12d8) at /usr/include/c++/4.4/bits/stl_list.h:805
#11 net::SocketMonitor::remove (this=0x811faf0, sock=0x86a12d8) at /home/rj45/Downloads/libktorrent-1.1.0/src/net/socketmonitor.cpp:170
#12 0x00331d6d in mse::StreamSocket::stopMonitoring (this=0x86e4158) at /home/rj45/Downloads/libktorrent-1.1.0/src/mse/streamsocket.cpp:114
#13 0x003406d6 in bt::Peer::~Peer (this=0x88cf438, __in_chrg=<value optimized out>) at /home/rj45/Downloads/libktorrent-1.1.0/src/peer/peer.cpp:115
#14 0x00342ac6 in qDeleteAll<QList<bt::Peer*>::const_iterator> (this=0x828fca8) at /usr/include/qt4/QtCore/qalgorithms.h:322
#15 qDeleteAll<QList<bt::Peer*> > (this=0x828fca8) at /usr/include/qt4/QtCore/qalgorithms.h:330
#16 bt::PeerManager::closeAllConnections (this=0x828fca8) at /home/rj45/Downloads/libktorrent-1.1.0/src/peer/peermanager.cpp:342
#17 0x00344b3f in bt::PeerManager::stop (this=0x828fca8) at /home/rj45/Downloads/libktorrent-1.1.0/src/peer/peermanager.cpp:437
#18 0x00381386 in bt::TorrentControl::stop (this=0x828f770, wjob=0x854a140) at /home/rj45/Downloads/libktorrent-1.1.0/src/torrent/torrentcontrol.cpp:499
#19 0x00f4951c in kt::QueueManager::stopSafely (this=0x8278db8, tc=0x828f770, wjob=0x854a140) at /home/rj45/Downloads/ktorrent-4.1.0/libktcore/torrent/queuemanager.cpp:747
#20 0x00f4a40b in kt::QueueManager::onExit (this=0x8278db8, wjob=0x854a140) at /home/rj45/Downloads/ktorrent-4.1.0/libktcore/torrent/queuemanager.cpp:424
#21 0x0806e85f in kt::Core::onExit (this=0x827b660) at /home/rj45/Downloads/ktorrent-4.1.0/ktorrent/core.cpp:912
#22 0x08076ba1 in kt::GUI::queryExit (this=0x8273b90) at /home/rj45/Downloads/ktorrent-4.1.0/ktorrent/gui.cpp:568
#23 0x012a3216 in KMainWindowPrivate::_k_shuttingDown (this=0x8285140) at ../../kdeui/widgets/kmainwindow.cpp:1142
#24 0x012a7ca6 in KMainWindow::qt_metacall (this=0x8273b90, _c=QMetaObject::InvokeMetaMethod, _id=7, _a=0xbfb4d5ac) at ./kmainwindow.moc:102
#25 0x012f052a in KXmlGuiWindow::qt_metacall (this=0x8273b90, _c=QMetaObject::InvokeMetaMethod, _id=38, _a=0xbfb4d5ac) at ./kxmlguiwindow.moc:102
#26 0x001355ca in KParts::MainWindow::qt_metacall (this=0x8273b90, _c=QMetaObject::InvokeMetaMethod, _id=38, _a=0xbfb4d5ac) at ./mainwindow.moc:75
#27 0x0807a0df in kt::GUI::qt_metacall (this=0x8273b90, _c=QMetaObject::InvokeMetaMethod, _id=38, _a=0xbfb4d5ac) at /home/rj45/Downloads/ktorrent-4.1.0/build/ktorrent/gui.moc:102
#28 0x01aa28ca in QMetaObject::metacall (object=0x8273b90, cl=QMetaObject::ReadProperty, idx=38, argv=0xbfb4d5ac) at kernel/qmetaobject.cpp:237
#29 0x01ab56ad in QMetaObject::activate (sender=0xbfb4d6a0, m=0x1bcf484, local_signal_index=0, argv=0x8be58955) at kernel/qobject.cpp:3280
#30 0x01b06c57 in QCoreApplication::aboutToQuit (this=0xbfb4d6a0) at .moc/release-shared/moc_qcoreapplication.cpp:131
#31 0x01aa0036 in QCoreApplication::exec () at kernel/qcoreapplication.cpp:1014
#32 0x05670e07 in QApplication::exec () at kernel/qapplication.cpp:3672
#33 0x08069011 in main (argc=1, argv=0xbfb4dad4) at /home/rj45/Downloads/ktorrent-4.1.0/ktorrent/main.cpp:177

Ok, I also reported this to bugs.kde.org as Bug # 270846
https://bugs.kde.org/show_bug.cgi?id=270846