I thought this might be what you were referring to It is from one of the Hoffnung Concerts " Concerto Popolare is that the orchestra believes it is playing Tchaikovsky's First Piano Concerto, but the pianist believes he or she is playing the Grieg Piano Concerto." It becomes a battle between Orchestra and Soloist and wrings in several other pieces of Music, all to great and smooth effect.
http://www.youtube.com/watch?v=PVC1AkIJh68