Fire at a fireworks factory = lots of explosions. I went to a fireworks display when I was a kid and about 5 minutes in, the structure they were launching them out of caught on fire and the rest of the display went off in the space of 2-3 minutes. Best fireworks I've ever seen...
Update: Wikipedia has more on this explosion, which occurred in The Netherlands in 2000. (thx to the dozens who sent this in)