If you are seeing Mongo soft rlimits warnings in your logs, or a
WARNING: soft rlimits too low. Number of files is 256, should be at least 1000 when you login to mongo shell via
mongo from the commandline, or any mysterious/unexplained mongo connection errors... follow this how-to exactly and it will resolve the issue for you.
(Source of this how to found at https://github.com/basho/basho_docs/issues/1402)
sudo vi /Library/LaunchDaemons/limit.maxfiles.plist