One exceptionally enjoyable play I went to see was supposed to be a moderately serious two-hander play, but at one point an actor said something like "the phone's ringing", walked to answer it, and thereupon it started to ring. He ad libbed something like "it's best to wait until it rings before doing that", found the audience enjoyed it, then the rest of the play turned into a bravura ad libbed performance vaguely related to the script. Great fun, but a little tough of the guy working the sound and lighting systems.