The mighty fall ALL the time. Amost every "folk icon" I've ever heard has forgotten words. I think if you perform, you should try your hardest to not do that, but realize mistakes are inevitable. What impresses me about the people I respect is what they do with the mistakes. They improvise on the spot, they sing another verse, they sing "la la la," they stop and remember, they ask the audience (if it's a well-known song) - in short, they keep going.