I am observing the twisted behaviour which seems rather strange to me. In short: if lineReceived returns some deferred, then the whole connection is aborted, moreover, this deferred is returned as error message. Longer story (the code pieces extracted from quite large app, I have not yet tried writing small example) ### Crucial code fragments class SomeProtocol(basic.LineReceiver): (...) def lineReceived(self, line): (...) return defer.succeed(997) # magic value I can recognize easily class SomeFactory(protocol.ReconnectingClientFactory): (...) def clientConnectionLost(self, connector, reason): logger.warn("Connection lost. Reason: %s\nTraceback: %s" % ( reason.getErrorMessage(), reason.getTraceback())) protocol.ReconnectingClientFactory.clientConnectionLost(self, connector, reason) reactor.connectTCP(HOST, PORT, SomeFactory(...)) reactor.run() ### Things seen in log file myapp : WARNING Connection lost. Reason: <Deferred at 0xB551E1ECL current result: 997> Traceback: Traceback (most recent call last): Failure: <type 'instance'>: <Deferred at 0xB551E1ECL current result: 997> (you recognized this 997, didn't you?) ### Extra info 1. Removing return defer.succeed() from lineReceived helps. 2. I wanted to return deferreds from lineReceived because I wanted to write unit tests testing my factory object (and in some cases to check whether all worked correctly required waiting for some spawned deferred calls).