"May be true nowadays, but certainly wasn't once."

It only happens when you *don't* use a lyric sheet - I do it at home all the time when I'm going through the process of learning the story behind a song. I sing along to myself, improvisiong verses where my memory for the lyrics is faulty. In public it happens when you need to improvise a lyric to the story, when you've muffed the lyrics you've memorised. It's much easier to do in private than in public thougfh!