When used as part of sharding, the ReplicaSetMonitor causes the config.shards entry for a particular replica set shard to be kept up to date as the replica set's membership changes.

If for whatever reason parsing the isMaster response fails, the RSM will consider the replica set as containing no hosts and will write a bad entry to config.shards and that entry will eventually be read by all other shards and mongos instances, rendering the shard unusable until manually repaired. This was evidenced in SERVER-27793 and the specific error is in this comment.

In order to be on the safe side, if the RSM fails to parse the isMaster response for any reason, it should log a warning, but not cause update of config.shards.