The omission of 'Bramble Briar' wasn't a 'mistake'; there's no reason to think that Child had ever heard of it (no broadside editions survive, and no example from oral tradition was published until after his death, unless you count a rather poor re-working in a Wehman songster of about 1890). He was aware of 'Polly Vaughan', but only via Jamieson and perhaps a few rather garbled late C19 broadsides. As things stood, it didn't fit his criteria, and its omission is not surprising. 'Long A-Growing' is one of the very few omissions that he might reasonably be taxed with.