Barenboim and Mehta: Were they on the same page?

An intriguing question from Rodney Greenberg, director of live concerts on British TV:

There’s a story about Mehta partnering Barenboim in a Beethoven concerto. The jet-setters haven’t had time to rehearse and Barenboim arrives at the concert just in time. He settles at the piano and looks at the maestro. Mehta stares back. Long silence. Barenboim frowns, gets up, looks at the score on the conductor’s stand. It’s Beethoven Number 4. Says ZM: “So? I was waiting for you to start.” DB: “But I thought tonight we’re doing Number 3?”

Can anyone say:

[1] Whether this actually happened or is it just a good yarn?
[2] If it’s true, how was it resolved? [Clue: the conductor’s score being No. 4, so were all the orchestral parts in front of the players. And… DB has a fabulous memory].