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 basho/basho_docs#1402)
First file:
sudo vi /Library/LaunchDaemons/limit.maxfiles.plist
...containing: