Mozart vs. Muzio Clementi
On Christmas Eve, 1781, Clementi and Mozart, who were friendly at that time, found themselves in a duel. The Emperor called a draw, and they were both required to improvise on a theme that the Emperor chose on the spot. After the duel, both Mozart and Clementi agreed afterward that Mozart won, but apparently, they were ambiguous statements.
Source: Bachtrak
Clementi was just being polite. Mozart wrote, “Clementi is a charlatan, like all Italians. He marks a piece presto but ‘plays’ only ‘allegro.’” Clementi said about Mozart: “Until then I had never heard anyone play with such spirit and grace. I was particularly overwhelmed by an ‘adagio’ and by several of his extempore variations for which the Emperor had chosen the theme, and which we were to devise alternately.”