I like the idea of this thread, but have so many anecdotes to tell. I will start with just the one, fresh in my mind since I just watched Barenboim's Schumann Concerto. He was playing it in a concert once and the orchestra started when he was not ready in the first movement, so he mucked up the opening passage a bit. In the second movement, he thought he would get back at the conductor and started with those four chords before the orchestra were ready. Needless to say since then conductors always checked that he was ready to go,Ed