When running the v2.8 release candidates log lines such as:

in rc3, as reported by customer:

mongos collstats doesn't know about: capped
mongos collstats doesn't know about: wiredTiger

or in rc4:

mongos collstats doesn't know about: paddingFactorNote
mongos collstats doesn't know about: indexDetails

have been observed when the db.<collection>.getShardedDistribution() or db.runCommand({"collStats": "<collection>"}) commands are run in the mongo shell.