I'm sometimes surprised when a singer forgets the words and then gets so wrapped up in their embarrassment that they don't realise there are others who konw the song and will help them thorough it.

And I agree with the vital point that there's a huge difference between forgetting the words and not attempting to learn them in the first place. Using some sort of crib-sheet as a security blanket, just there if you blank out, is a world away from reading every word from a file and turning the page back to read the chorus every time.

And what difference does it make, some are asking? The difference, in most cases, between an enjoyable and engaging performance and a stilted, boring rendition of a song that seems to mean little or nohting to the singer. You have to sing the song as if it matters, and if you don't appear to know the words at all, the impression you give is that it doesn't matter, which turns many of your audience right off.