Mozart’s sense of mischief is evident in his behaviour at a performance of The Magic Flute.  Thisis from a letter to his wife Oct 8 & 9 1791:

… (1) had a box [in the theatre] this evening and applauded everything vigorously; but He, that Know-it-all, proved to be a real Bavarian; I couldn’t stay with them or I would have been tempted to call him an ass; – unfortunately, I just happened to be in their box when the 2nd act started, and it begins with a solemn scene. – He laughed at everything; at first I was patient enough to draw his attention to some of the lines, but – he just laughed; – well, it was too much for me – I called him a real Papageno and left the box – but I don’t think this Nitwit understood what I meant – so I went to another loge, occupied by Herr Flamm and his wife, and there I could enjoy the opera fully, and I stayed till the end; – except when Papageno’s aria with the Glockenspiel came on, at that moment I went backstage because today I had a kind of urge to play the Glockenspiel myself. – So I played this joke: just when Shickaneder [playing Papageno] came to a pause, I played an arpeggio – he was startled – looked into the scenery and saw me – the 2nd time he came to that spot, I didn’t play – and this time he stopped as well and did not go on singing – I guessed what he was thinking and played another chord – at that he gave his Glockenspiel a slap and shouted “shut up” – everybody laughed. – I think through this joke many in the audience became aware for the first time that Papageno doesn’t play the Glockenspiel himself.

(1) A name was crossed out.

Cited in: Spaethling, Robert (2000) Mozart’s Letters; Mozart’s Life.  London: Faber and Faber, p.441.