Inside a synthetic fibre factory, a leak estimated at 4,000 litres of acid solution, containing 200 kg of hydrocyanic acid (HCN), occurred within an acrylonitrile production unit. This leak was situated at the level of a pipeline running between a condenser and a separator on installations operating at 30° to 40°C under atmospheric pressure. The absence of a flange on a pipeline tap caused this accident. The investigation indicated that this condition was present throughout the week leading up to the leak. The tap had likely been plugged by the polymerised product. Since the raw material tank had been filled to a high level, the pressure surge occurring in the tank was sufficient to loosen the plug and cause discharge of the fluid. The site operator installed a water curtain to prevent hydrocyanic acid (HCN) from spraying beyond the factory boundaries. Atmospheric HCN concentrations were frequently tested due to the proximity of dwellings (less than 500 m away). The operator proceeded by shutting down all HCN units. The faulty pipe segment was replaced and all similar pipes were inspected. Only one employee sustained slight HCN intoxication. According to the investigation’s initial findings, the leak had resulted from stress corrosion of the carbon steel pipeline, in the presence of an aqueous acid solution. The defective weld at the spot of the missing flange had been made 15 years prior. The weld heat treatment recommended at the time of construction had never been performed during any of the intervening repairs. After the accident, since the operator could no longer recall either the number or location of these repairs, all units underwent an inspection: 15 welds were identified as post-construction, and all displayed stress corrosion. The resumption of factory activity was contingent upon replacing the modified pipe segments by stainless steel lines. Based on feedback from this incident, the operator was required to introduce a database for recording repairs/modifications as well as scheduling inspections and maintenance visits.