Note that this happened to me, Doc Cathode, personally. It is not an email or UL altered for dramatic purposes. I did not see it in a fim or hear about it on the news. It happened to me just as it is described here.
2 years ago, I was happy to discover a bag of fireworks in one of my cabinets. I do not go in for the risky ones. There were charcoal snakes, sparklers, morning glories, a tank or two, a friendship pagoda, and a few cones. All of this was several years old. Some of the sparklers and morning glories wouldn’t light. I carefully doused these just in case.
One of the cones gave me a problem. The plan was to light the fuse, get away, and watch multicolored sparks shout about 3 feet into the air. I lit the fuse, got away, and waited. Nothing happened. Slowly, I approached and listened for sounds of combustion. There were none. I lit the excess paper at the cone's top and got away. Much more quickly than it should have, the cone went off.
There were no sparks. Instead, there was a very loud explosion and the whole cone was launched some 20 feet straight up. Bits of flaming cardboard and paper were strewn all over the parking lot.
Had the cone gone off a little earlier, I'm sure that I would have lost parts of my right hand.
I asked a friend with a chem degree about it. I don't remember the exact explanation he gave me. But, he said it happened to all similiar fireworks after awhile.
If you have old fireworks, douse them throroughly with water and dispose of them. They may have changed from something that produces lights and may burn to a chunk of explosive.