I have been hearing the same story (with only the names of the concertos changed) since 1967.

Me too. Then it was a violinist and they were playing the Brahms and Mendelssohn concertos in the same concert. He thought it was Brahms first (which has a long orchestral intro before the soloist comes in); when the orchestra started playing he had exactly two bars of music to catch on and start playing the Mendelssohn as if nothing had happened.

But it's the sort of thing that could have happened more than once...