A few years ago, there was an explosion in a reactor at the XYZ Chemical Company. The reactor was a block copolymerization reactor where butadiene and styrene monomers were added in a semibatch manner The heat of reaction was removed by means of a refrigeration system. The incident occurred when the reactants were not added in the sequence or quantity required. The resulting runaway reaction overpressurized the reactor system and caused an expansion joint to rupture. The hydrocarbons released formed a vapor cloud that exploded, causing the death of two workers, serious injury to three others, and significant damage to the facility. The contributing causes of the incident include: (a) lack of information about the hazards of the chemicals in the process, (b) lack of information about the equipment in the process and inadequate equipment; (c) inadequate or no PHA, (d) inadequate training, (e) confusing operating procedures, (f) failure to use management of change procedures, (g) failure to perform a prestartup safety review, (h) failure to have an adequate contractor safety program, and (i) failure to investigate previous near misses and incidents and take corrective actions. This article describes this accident, the root causes, and the major plant deficiencies. The following expresses my opinion of the events that led to this explosion as testified to in open court. Others may have different opinions. (C) 2009 American Institute of Chemical Engineers Process Saf Prog 29: 144-149, 2010