From Nathan Milstein's From Russia to the West:
"Toscanini's legendary temperament, well known to the musicians and the public, exploded once in my presence in a very funny way. In New York in 1939, a benefit was held for the Chatham Square Music School. The star attraction of the program was the "children's orchestra", as it was advertised. The orchestra's violinists included Heifetz, Adolf Busch, Oskar Shumsky, and me, and I remember that one of the violists was William Primrose. The cellists included Emmanuel Feuermann, Piatigorsky, and Alfred Wallenstein.
All eighteen or twenty of us "children" came out onstage in shorts, even Busch, a big, red-faced man who was almost fifty then, yet looked younger than the rest of us.
The conductor of our "children's orchestra" was none other than Toscanini. We played Ferdinard Rie's "Perpertuum mobile." Heifetz said before the performance, "Let's surprise the maestro! He'll be keeping a strict beat, as usual. But we'll start an incredible acceleration. Let's see how Toscanini reacts."
The maestro came out to his "children's orchestra" in a long, old-fashioned coat, like a school teacher. He began conducting and we followed Heifetz's plan and played faster and faster. Toscanini could not understand what was going on! He was so angry that he dropped his baton and ran off!
The "children's orchestra" was a great success; the audience loved it and thought that the trick with the tempo had been planned that way. The maestro, however, was furious and would not come out for a bow. Instead Wanda [Toscanini's daughter] came out, dressed in a man's suit (she had borrowed my pin-striped pants) and with a hat in her hand. She even twirled a drawn mustache, as if she were the maestro. The audience was certain that this too was part of the grand design and went wild."