Obviously, traditional songs come in many variants, so it's a mistake to suppose that a particular version is wrong just because we know another, different one.  On the other hand, there are a lot of transcriptions here of specific recordings, generally made by ear, which sometimes contain quite ludicrous errors.  It's always worth pointing those out, but only, of course, if you are really sure that you're right; I've known people to argue at great length that their particular hearing of a phrase must be right when it's clearly nonsense, sometimes to the extent of concocting exotic false etymologies for words which they think they've heard, and which make no sense at all unless a completely new meaning is invented for them.  Sometimes, too, people get quite upset when it's suggested that they, or someone they admire, may perhaps have made a mistake.

It's a minefield, in short.  I do point out errors of transcription from time to time, since there's a very big difference between a traditional singer making an inadvertent change in a lyric, and somebody writing down what they think they've heard on a record: the first may be considered as part of a legitimate "folk process", while the latter is just a mistake.  Mind you, I've been here a couple of years, now, so they're getting used to me.  If you have useful information to add, then please do it; but do also check around to make sure that it hasn't already been done, and try to be gentle about it; the "Greenland Whale Fisheries" example that Joe referred to above, for instance, came from someone who evidently knew a great deal about nautical terminology, but very little about folksong.

Malcolm