In pymongo 3.6.1 its possible to get this exception:

Traceback (most recent call last):
  File "/opt/venvs/mongo_source/lib/python3.5/site-packages/pymongo/pool.py", line 952, in _get_socket_no_auth
    sock_info = self.sockets.pop()
KeyError: 'pop from an empty set'

I can replicate this easily by running inside of an environment with large packet loss and a client set with a short timeout: `pymongo.MongoClient('mongo', j=True, socketTimeoutMS=10)`