When using a connection string with multiple servers in a replicaset, Ping() does not detect if one of the MongoDB instances go offline, it will not throw an exception.
Only if I call Reconnect() will the driver detect that one of the instances are offline and throw an exception when calling Ping().

An easy way to test is to put one of the MongoDb instance hostnames in your hosts file and point it to 127.0.0.1, assuming you don't have mongodb running on your localhost.