There's a story of an ex hausted tenor at La Scala who, facing repeated cries of "Encore" responded that he couldn't go on.
A man rose in the audience to say, "You'll keep singing until you get it"
Source:
http://www.nypost.com/p/news/national/obama_answer_for_america_more_of_1IboSZ3t6WFW12qZnpv10J
