An exothermic runaway chemical reaction abruptly occurred within a production facility when an operator initiated an automated sequence to add water inside a multi-purpose batch reactor. The reactor suddenly experienced a pressure build-up due to its foaming content, but the automated control system was not able to regulate it. The glass reflux condenser burst while the rupture disc remained unbroken, given that its bursting pressure had not been reached. The irritating HCl vapours released in the facility are evacuated outside by an employee through the ventilation system. These vapours intoxicated 7 people in the neighbourhood, 2 of whom would be kept in hospital overnight for observation.The investigation conducted revealed that the sudden exothermic reaction followed a 30-litre spill of water into the reactor instead of the 3-litre quantity indicated in the procedure. The 32-litre water tank was connected to the reactor via a pipe fitted with 2 valves. The first valve, activated by the automated control system, normally delivers 3 litres of water, while the second manual valve is supposed to stay in the closed position at the beginning of this water addition sequence. During the accident however, the manual valve was left open, and this oversight led to quickly draining the 30 litres water content of the tank into the reactor. The exothermic reaction was triggered, and the sole control system designed to prevent chemical runaway was unable to function properly, since the system was designed to provide a control of the water added to the reactor through the closure of the second valve. Moreover, the manual valve, which was not equipped with an open/closed position indicator, could not be easily reached by the operator, making it difficult to control. This risk of exothermic reaction had been identified during the process safety study (i.e. the Hazard and Operability Study, or HAZOP), but at the time of the accident only a call for procedural improvements had been issued. The company limits the maximum volume of water which can be added at one time in the reactor and improves the automated control system. The safety studies (HAZOP) are updated for all exothermic reactions carried out on in this apparatus. A particular attention is paid to consequences of operating failures and a balance is found between risk control measures and the potential severity of the consequences.