I have a fast cron job running in 5min intervals:

conn = MongoClient(...)

1. quick find() lookup, using indexes
   conn.close()

About once a week I receive traceback error:

Traceback (most recent call last):
File "/usr/lib64/python2.7/atexit.py", line 24, in _run_exitfuncs
func(*targs, **kargs)
File "/usr/lib64/python2.7/site-packages/pymongo/periodic_executor.py", line 142, in _shutdown_executors
executor.join(1)
File "/usr/lib64/python2.7/site-packages/pymongo/periodic_executor.py", line 81, in join
self._thread.join(timeout)
File "/usr/lib64/python2.7/threading.py", line 939, in join
raise RuntimeError("cannot join thread before it is started")
RuntimeError: cannot join thread before it is started

MongoDB is using replica set, if that matters...