In 2004, a thermal power station experienced a serious leak of anhydrous ammonia while carrying out a mandatory periodic test of a manifold multiport type pressure safety valve. The device incorporated an additional relief valve which allowed removal of any one of the relief valves for servicing without the need to depressurize the vessel. Power station staff were unfamiliar with the device and subcontracted the task to the valve vendor, who in turn subcontracted the job to a specialist contractor. During the procedure, a relief valve separated forcefully from its 2" nozzle resulting in an uncontrolled release from the 400 m³ ammonia storage tank. Members of the public 500 m away were exposed to ammonia vapour. The investigation identified a weakness in the design of the device with allowed erroneous installation of an indicator that was intended to show which of the relief valves could be removed safely. The subcontractor showed a lack of understanding of safety critical details of the valve design. The incident offers several important accident prevention lessons: 1) A valve design vulnerability unknown to all parties involved; 2) Safety systems may be unavailable due to common mode causes (before the job, automatic water curtains to absorb ammonia vapours were disabled); and 3) Knowledge/competence loss when subcontracting. The incident has considerable learning potential and should interest industry and safety professionals due to the widespread usage of the multiport device at major hazard sites (LPG, ammonia etc.). It should interest policy makers because the hazard (ammonia) had been introduced for environmental reasons (flue gas deNOx) and because principles of inherent safety were ignored – less toxic alternatives to ammonia exist. The unavailability of information on this incident from open sources however, suggests that the process of learning from past incidents is challenged.