I think most of us can think of some cringe-making slip-ups we've made, and having a word-sheet there would be no guarantee that this wouldn't happen. Like, for instance, when I sing, "The bird's in the nest, the trout's in the burn ..." the first time, and then at the repeat a few seconds later I can find myself singing "The trout's in the nest..." and I know the bird isn't in the burn but it's too late, and so that's where it ends up - or possibly I'll have two trouts, one in each location.

Anyway, just think how dull it would be if we never had these bloopers, only ever the exact wording that's on the page! IME it tends to happen more when you've stopped concentrating on the song, or are distracted by something happening in the room. If you're really thinking about the words and the story, you're less likely to go wrong. (THINK: What's in the nest, a trout or a bird?) And as you say, the mistake is almost always more noticeable to the performer than to the audience, as long as you can keep going as if nothing was wrong.

Marje