The "source" singers who people keep referring to got their material from earlier sources. I'm sure that very few of them felt that they had to go back to an earlier version to find out if they'd got it right - even if they'd had the time and resources to do so. I'm also certain that many of these "source" singers would have found nothing wrong in a bit of judicial editing here and there. The "Mondegreen Effect" is also very evident when you compare versions of songs, as well as the replacement of some "foreign" dialect words with more local ones. In fact it is these little changes and variations that have contributed to the wealth and variety of our oral tradition.

Nowadays we have much more time and resources (Internet for one) to search for earlier versions of songs and it's interesting and useful to look at these and sometimes change our own versions accordingly where we think that it improves it in some way. However, perhaps "natural selection" appplies to folk song as well. If songs are not allowed to adapt to different times and customs perhaps many of them will cease to be sung.

Interestingly as the result of another current thread here, John Dudley of the Copper Family has discovered that they sing a word wrong in "The Innocent Hare". He reckons that as it's been wrong for probably three generations, they'll stick with what they're singing - and I applaud him for that decision.