it is currently possible to perform manual/direct modifications to documents in config.shards on a live sharded cluster, even though this is not supported and has undefined consequences, including rendering the cluster inoperable. We should investigate if we want to make the system tolerant of such changes (and if so, in what way), or if they should be completely disallowed on running systems. (Note that it should probably never be disallowed completely, since it is sometimes necessary in order to fix unexpected issues.)