The driver allows a connection string with a number of hosts, but the hosts can belong to multiple replica sets.
eg. host1,host2,host3,host4,host5,host6 where 1-4 were one replica set (2 being primary) and 5-6 another replica set, with a different name, and 5 being this sets primary.
The situation occurred when we used two nodes to create another replica set, but failed to change the connection string.
No error's were reported by the driver, instead strange application behaviour was experienced as sometimes nodes 5 and 6 were queried, but contained no relevant data.
I know there is a replicaSet option, which I believe causing more checking, but I would think that the default behaviour would be to check that all hosts were in the same replica sets, and to only be able to see one primary.