This review of a Seattle Symphony concert from 2007 mentioned that back in 1998, soloist Jon Kimura Parker inserted the theme from The X-Files into one of his cadenzas. Cadenzas were originally points in a concerto at which soloists could improvise and show off their technical skills, but over the years, the contents of cadenzas have become more and more rehearsed, with most composers having switched over to fully-written-out cadenzas over a hundred years ago.
That's why it's so exciting when an improvised cadenza reappears on the scene. All of a sudden, anything can happen. Reading the article reminded me of a cadenza surprise in a performance by Awadagin Pratt nearly twenty years ago. I'm fairly certain the piece was a Mozart piano concerto, and during the cadenza, Pratt slipped into a middle voice the theme from the opening movement of the Mendelssohn Scottish Symphony, which was the next piece on the program! By the time I figured out what was happening, it was over. My opportunity to burst into applause in the middle of a piece was lost.