The Oppau explosion occurred on September 21, 1921 when a tower silo storing 4,500 tonnes of a mixture of ammonium sulfate and ammonium nitrate fertilizer exploded at a BASF plant in Oppau, now part of Ludwigshafen, Germany, killing 500–600 people and injuring about 2,000 more. The plant began producing ammonium sulfate in 1911, but during World War I when Germany was unable to obtain the necessary sulfur, it began to produce ammonium nitrate as well...
Compared to ammonium sulfate, ammonium nitrate is strongly hygroscopic, so the mixture of ammonium sulfate and nitrate clogged together under the pressure of its own weight, turning it into a plaster-like substance in the 20 m high silo. The workers needed to use pickaxes to get it out, a problematic situation because they could not enter the silo and risk being buried in collapsing fertilizer.
To ease their work, small charges of dynamite were used to loosen the mixture. The procedure was tried experimentally and was considered safe; it was not known at the time that ammonium nitrate was explosive. Nothing extraordinary happened during an estimated 20,000 firings, until the fateful explosion on September 21. As all involved died in the explosion, the causes are not clear. A theory is that the mixture changed and a higher concentration of ammonium nitrate was present.Oops.
In my searching, I found a rather amusing early news report from Nature on the explosion, which includes this gem:
...in substantiation of its innocuous character they adduce the fact that in factories producing it no accident has occurred for a number of years, when explosives have been applied to it for the purpose of breaking up blocks of the mixed salts which have set hard.I'm fairly new to this field, but it seems to me that part of process chemistry is showing "the box" in which a process can be run. A process chemist can only hope to guarantee desired results (yield, purity/quality, safety) in within the reaction parameters (heat, concentration, volume, what have you) that have already been defined. One wonders what happened at Oppau; obviously, someone stepped outside the box.