Particularly newly discovered branch mint proofs in the Morgan series. Or more particularly, ones that don't yet have their own SP tier in any database but are obviously unique.
And if something like that is submitted to NGC, should the submitter apply "Variety plus" for closer inspection or would the general graders take some kind of special action?
With the amount of evidence that gets habitually destroyed by the US Mint in just the last 100 years, there would surely be pieces floating around today that don't necessarily have evidence to prove their existence.