'Fail a single TCP connection established on this
L{ConnectionCompleter}\'s L{MemoryReactor}.
@param reason: the reason to provide that the connection failed.
@type reason: L{Failure}'

def failOnce(self, reason=Failure(ConnectionRefusedError())):

    self._reactor.tcpClients.pop(0)[2].clientConnectionFailed(self._reactor.connectors.pop(0), reason)
