A couple weeks ago I had a happy accident. I'd loaded the camera with a roll of Wittner Chrome 200D slide film. I'd set the camera (A Pentax PZ-20) on aperture priority mode (I don't know why I didn't use Manual, but I digress) and set the aperture for the flash I was using (A Pentax AF200S) to correspond to the film speed I'd selected. However, I neglected to let the camera in on the film speed, so it assumed 100 speed film, since Wittner Chrome 200D isn't DX coded. Off I went to my son's graduation. I realized after I'd shot all the graduation pictures that the camera's aperture was set correctly for the flash (f/4 for ISO 200) but the camera was on aperture priority and thought it had 100 speed film.
The pictures came out pretty good, I think. It was mixed lighting (flash + mercury vapor lights in the gym) so the background color is a little off. What's awesome is there's all sorts of detail in the background, yet the foreground was properly exposed by the flash.