Sitting in the orchestra seats at the farewell concert of Alfred Cortot when he played an all Chopin programme (his 24 preludes and studies, if I recall correctly). The reception from the audience was ecstatic and Cortot was recalled time and time again. I think he must have played at least 5 encores. Sir Thomas Beecham conducting the Royal Philharmonic Orchestra. Seated in the orchestra seats his sing-a-long was very audible, but did not diminish the quality of the performance.