Hello,
I don't know is this a bug or a feature, but in fact it is very annoying thing.
If you try to connect to a MongoDB server with a wrong login\password or an another incorrect property that prevents establishing of a successful connection, an instance of MongoServer class will attempt to connect to the server while the process active.

var server = new MongoServer(anyWrongProps);
server.Connect(); // an exception should be raised here

After that the interminable loop

while (!heartbeatCancellationToken.IsCancellationRequested)

will be in progress in MongoServer.MonitorServerAsync() method.

I have tried to fix it with the following code

ClusterRegistry.Instance.UnregisterAndDisposeCluster(server.Cluster);

Seems it works, but perhaps it can be a reason of some problems. I don't know is this safe.

Best regards,
Alex