If add a replica set as a member of a shard, with a space between the comma and one of the hosts, like so:

db.runCommand(

{ addshard : 'shard0/localhost:4000, localhost:4001', name: 'shard0' }

)

I get a misleading error:

"can't use localhost as a shard since all shards need to communicate. either use all shards and configdbs in localhost or all in actual IPs host: localhost:4001 isLocalHost:0"

Better to trim spaces in lists of replica-set members, or to give a more informative error.