After performing maintenance on a distillation column in a chemical plant, a 80-kg leak of hydrogen cyanide (HCN) occurred on a buffer tank used to store water laden with traces of HCN. A recycling step carried out shortly before on this same vessel had been accelerated by opening the automatic transfer valve bypass. Upon completing this recycling step, the technician failed to close once again the corresponding manual valve, and HCN present at the base of the absorption column flowed back into the buffer tank connected to the column. The tank’s head valve (perhaps a relief valve?) then opened, enabling an HCN release into the atmosphere. Alerted by detectors installed inside the unit, employees were confined indoors. The leak was quickly observed by plant personnel, but it took 3 hours of investigation to identify the initial cause; during this interval, in-house first responders set up water curtains that, according to the operator, would absorb most of the HCN released. The accident had no noteworthy impacts on employees, neighbours or the environment. The operator decided to padlock the incriminated valve bypass, along with all similar critical devices throughout the facility (especially on the ammonia circuit) and organised technician information sessions. The possibility of installing check valves was also studied.