Even as he was preparing them to play the piece, I could tell he was a really super-nice conductor. Basically, he said, "The best way to do well is to look at where you will mess up, and then don't. But, if you do, no problem. We'll just stop and do it again. No pressure."
It's very good. But the best April Fool's Day prank was done by the BBC when their very highbrow, serious documentary programme had a report on the failure of the spaghetti harvest in Italy.