There's no checking if multiple database names are provided, I'm just starting with mongoDB with node.js and I fall for that one because of a typo that produced this URL:
mongodb://127.0.0.1/27017/test

The driver worked with no complains but returned nothing for my query because it created and worked on a new database named '27017' instead of database 'test

I checked already and that should URL should be violate the format:

mongodb://[username:password@]host1[:port1][,host2[:port2],...[,hostN[:portN]]][/[database][?options]]

So I guess an error should been thrown there, the fix is trivial and ready just waiting for confirmation.