Amidst an ongoing argument as to who was the greatest, between those who admired Niccolò Piccinni and those who admired Christoph Willibald Ritter von Gluck, the former was introduced to Marie Antoinette, who immediately offered to sing for him. Having accompanied the famous composer to the piano she chose a musical score. Only later did she realize that her selection was a piece by Gluck.
Edmond Guérard, Dictionnaire encyclopédique d’anécdotes (Paris: Firmin Didot, 1872), Vol. 1, p. 336. (Nobility.org translation.)
Short Stories on Honor, Chivalry, and the World of Nobility—no. 450