To answer Geoff's question from the standpoint of do any of us have a musical joke [ourselves, rather than knowing about one on youtube], here is one that was used by a musical group I performed with many years ago.
While you are playing one piece, you slip into another piece, ideally one that is more lighthearted than the first. Depending on one's personality (you have to be willing to look foolish), you can act as if you are just absent minded, or that the instrument has mind of its own, the sheet music is wrong, or something else that is not your fault. Since most popular music begins and ends on the tonic cord, you can switch from song to song endlessly. Switching from a somewhat serious song to a Christmas carol is a good way to start. You can then catch yourself and go back to the serious one, and soon enough drift off into a different Christmas carol. Victor Borge is a good role model for this sort of thing.