This causes the overall MR to fail with "not master and slaveOk=false" when trying to establish cursors on the shards, because the cursors are supposed to be against the primary but if there has been an election then the node the query is sent to might now be a secondary. It's not clear to me if the MR should just read from the secondary, or if this is just another failure mode of the legacy MR system (ie. it's simply not robust to any elections at any point during its runtime).