This video, shot earlier this year in Italy and uploaded today by a pyrotechnics enthusiast, shows what happens when an entire fireworks display accidentally goes up at once. To sum up: Holy shit. You'll want to turn the volume down a little bit for this one.
No one was killed, according to the video uploader, because the wall of vines in the foreground protected the audience from most of the shrapnel. Some of the people trying to escape the blast sustained scratches on the arms and belly, but the cameraman whose entire YouTube channel is devoted to fireworks, knew enough to take cover instead of trying to make a run for it.
Traditional Italian fireworks use open fuses that have to be lit manually, rather than the electrical fuses common in U.S. shows. That might make it more likely that a whole set of mortars could suddenly ignite at once.