Beethoven once gave a performance of a new piano concerto in which he forgot he was the soloist and began to conduct instead.  At the first sforzando he threw out his arms so vehemently that he knocked both candlesticks off the piano.  The audience burst out laughing, which enraged Beethoven.  He made the orchestra start all over again.  To avoid a repetition of the accident, two boys from the chorus were recruited to stand holding the lights on either side of Beethoven.  Unfortunately, when it came to the sforzando, Beethoven again flung out his arms.  One of the boys, keeping a wary eye on the maestro, had seen what was  coming and managed to duck.  The other boy was not so lucky, and received Beethoven’s hand full in the mouth, which made him stagger and drop the candlestick.  The audiences laughter now so enraged Beethoven that he hit the keyboard with his fist and broke half a dozen piano strings!

Cited in Kelly, Henry & Foley John (1998) Classic FM Musical Anecdotes.  London: Hodder & Stoughton, p.200.