Fun stories here (from Kliph Nesteroff, link from Mark Palko).
One of my first ever blog posts was about plagiarism in the performing arts (here FWIW). In written work, the author can footnote and give credit where due. The performer cannot. Besides, the audience doesn’t care. They want the comedian to make them laugh, and they don’t care much if someone else — uncredited and unpaid by the performer — wrote the jokes.