Forgetting the words is the crucial thing. And the tune. That's how the edges get robbed of and the odddities introduced.

If your memory isn't what it was a single person can do quite a llt of that unaided. It's one reason old people are so important in the folk process.

Where there's a live tradition it's quite possible for a new song to emerge which is totally part of the folk trdaition from day one. It'll still probably benefit from knocking around a bit.