In my case, I was deploying a Django app with Gunicorn with a `gevent` worker type running on CentOS 7.2 with Python 2.7.5. Unfortuantely for us, that combination of software triggers a regression in `urllib3` with `gevent`. See issues #482 and #702. The troublesome thing about this issue is that it will only show in a full production configuration.
As of this writing, my only workaround has been to remove gevent as the worker type.