Comment 1 for bug 175572

It seems we are just not catching and suppressing the client disconnecting. We already trap for
                    except self._socket_timeout:
                        # just check if we're asked to stop
                        pass
                    except self._socket_error, e:
                        # if the socket is closed by stop_background_thread
                        # we might get a EBADF here, any other socket errors
                        # should get logged.
                        if e.args[0] != errno.EBADF:
                            trace.warning("listening socket error: %s", e)

I suppose it is also possible that it is the SmartServerSocketStreamMedium which needs the exception trapping.