...during Papageno's aria with the glockenspiel I went behind the scenes, as I felt a sort of impulse today to play it myself. Well, just for fun, at the point where Schikaneder has a pause, I played an arpeggio. This time he stopped and refused to go on. I guessed what he was thinking and again played a chord. He then struck the glockenspiel and said 'Shut up'. Whereupon everyone laughed. I am inclined to think that this joke taught many of the audience for the first time that Papageno does not play the instrument himself.
(Mozart to Constanze, October 8-9, 1791.)