A spent caustic tank exploded in a major Midwestern USA refinery on August 14, 2007. The explosion occurred while the operators were transferring spent caustic into the atmospheric-relief, cone roof storage tank that had been in service since 1956. Fortunately, there were no injuries or fatalities. Property damage was, however, sufficient to interrupt the spent caustic disposal process. Unit operation was restored by commissioning an alternative, inherently safe spent caustic storage system. An investigation was launched to determine the explosion's cause. This article details the investigation's conclusions and corrective actions. It describes how a 2005 Hazard and Operability action item's intended purpose to reduce the risk for a PSM failure was defeated by equipment design errors, an unauthorized procedure deviation, and a series of management system failures. Lessons learned are communicated for organizations to avoid repeating similar mistakes. (C) 2010 American Institute of Chemical Engineers Process Saf Prog 29: 332-339, 2010