You may find yourself in the situation where when configuring nagios to monitor a MongoDB, it will complain that:
"CRITICAL - Unable to find host XXXXXXXX in replica set."
And you know for a fact that the replica set is fine. What has gone wrong then?
You have told nagios to monitor the IP address of the host in question, where in the replica set the host is found by its DNS name. Currently nagios-plugin-mongodb does not take this into account.