Here's an example of the sort of memory trap that can arise in folk songs. "Bold William Taylor" is about a girl who disguises herself as a man and joins the army to seek her own true-lover. She is discovered when a costume malfunction reveals her to be a woman. The following verse begins:

"Then the sergeant stepped up to her
Asking what had brought her here"

Several verses later, after she's found (and shot) her own true-lover, the same formula appears:

"Then the captain stepped up to her
Pleased well at what she'd done"

One time, I got them muddled and sang:

"A silver chain pulled down her waistcoat
Did expose her lily-white breast

Then the sergeant stepped up to her
Pleased well at what she'd done"

I'm sure he was, too!

Now there is a danger that this sort of mistake gets "embedded" so you keep on repeating it. However, I now have it in my mind as a potential problem, so that whenever I sing it I concentrate at those verses to make sure I sing the correct couplets. I've never made that mistake again.

Now perhaps some will say that wouldn't have happened if I'd had the words in front of me. However, I think I perform better without them, even with the occasional mistake. And having the words is no guarantee - it's very easy to glance away and then lose your place on the page.

Besides, I don't think many in the audience noticed that mistake. I don't recall a reaction - no wave of titters going around the room, no outbreak of grinning. People are listening to the story, and by the time a mistake has registered the story has moved on. Mistakes are nearly always worse for the performer than the audience, so it doesn't do to get to hung up about them. Learn from them.