When intentional, an ad-lib is usually used for a topical reference in a comedy. When a touring company reaches a new city, it will usually add a line of reference to something specific about that city. It is also an improvised line for added effect, again in comedy, or to cover when another actor dries. There are occasional instances when almost the whole performance has been (unintentionally) ad-libbed - as in a 1970s New York staging of Beckett's Endgame, where repetition in the text led Joseph Chaikin to confuse one of Clov's early lines with one of the closing lines of the play, and the remainder of the performance was spent attempting to get back to the correct place in the script.
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