If you do something as simple as Mongo() you will not get an error if the server isn't running until you run a command or query. (isMaster is no long called unless the pair/list constructor is used)

This is a byproduct of the new replicaset discovery and the ability to connect directly to a non-master node therein. We should probably put the immediate error trigger back in when constructing a Mongo instance. To do this I suggest adding a constructor (or set) that takes a bool doNotAddAllNodes, or something like that.

Right now the constructors are not very helpful in describing the expected behavior. Does the driver find the master or other slaves/nodes? If so which node is actually used?