As Richard quite rightly states the OP is referring to errors rather than adaptations, the one accidental or even careless and the other deliberate.

However this is also the essence of the oral tradition and most of those who love songs that have been shaped by oral tradition feel that the overall effect is largely one of improvement.

Even taking Don's example. He abhors the much corrupted Billy McGee McGaw descendants of The Three Ravens. I prefer to think of these as separate songs with different purposes. Thay have become comic songs and much of their comedy lies in the method of performance. I am in the advantageous position of being able to appreciate both songs and indeed others on the same evolutionary path.