The absolute worst mistake of this sort I ever heard was Jesu, Joy of Man's Desiring played on my university's carillon. What were written as short notes just hung on forever, so that it sounded like somebody'd dropped the whole instrument down the stairs.
My worst bagpipe experience is hearing some @#%$ play the theme from Star Wars. At a "medieval" event. Why nobody chucked bricks at him, I'll never know.
And then my father told me about somebody he met who had a summer job playing Flight of the Bumblebee on the tuba at an amusement park. Hmmm.