We recently started experiencing the following error: *RuntimeError: maximum recursion depth exceeded while calling a Python object* After a fair bit of debugging we eventually found the problem and implemented a fix for it in our codebase. We found that if a waiting queue on a DeferredLock is ~125 (depending on call depth) the error can error. I recommend releasing the lock in a callLater. The offending line is here: https://twistedmatrix.com/trac/browser/trunk/twisted/internet/defer.py#L1360 And the script below can be used to reproduce the issue. #!/usr/bin/env python from twisted.internet import defer, reactor LOCK = defer.DeferredLock() FIRST_RUN = True def deferred_sleep(delay): d = defer.Deferred() delay = reactor.callLater(delay, d.callback, True) d.delay = delay return d @defer.inlineCallbacks def synchronized_call(): global FIRST_RUN yield LOCK.acquire() try: if FIRST_RUN: FIRST_RUN = False yield deferred_sleep(2) print "Queue length: %s" % len(LOCK.waiting) finally: LOCK.release() #reactor.callLater(0, lambda: LOCK.release()) if __name__ == '__main__': count = 125 def start_test(): for i in xrange(count): synchronized_call() reactor.callWhenRunning(start_test) reactor.run() <http://www.ml.sun.ac.za/>