A secondary in a replica set was sent a QueryMessage with the SlaveOk flag set, and it responded with the QueryFailure flag set and $err = "not master". Apparently "not master" can be returned for multiple reasons (not just because a query is sent to a secondary without setting the SlaveOk flag); for example: if the secondary is in recovery mode it responds to queries with "not master". It would be helpful to the caller (and/or driver) to be able to tell the difference between a real "not master" and other things blocking a query like "in recovery mode".