If one has been running as a replica set member, and then restarts with --master, mongod gives no warning and just goes at it, creating the non-replset local.oplog.$main collection.

If on purpose that would be semi-ok but it is likely in such as scenario to be user error so we should do something either 1) warn, 2) disallow, or something.

If disallowed we should tell people how to do this on purpose – for example, backing up and then deleting their local database before starting back up with --master would be one approach. --master is deprecated so a warning might suffice.