A chlorine dioxide (ClO2) solution, used to disinfect water supply through the network, flowed at night inside a drinking water production plant. The next morning, employees discovered a major leak on the ClO2 production reactor from the presence of gaseous chlorine (Cl2) and soda. They attempted to neutralise the escaped ClO2 by introducing sodium bisulfite, which released chlorinated vapours inside the room. Personnel bothered by these vapours were forced to leave the premises. Fire-fighters equipped with self-breathing apparatuses closed the reactor feed valves to stop the flow (at a rate of 500 litres/h), ventilated the room and cut off the electrical power supply so as to avoid all risks of explosion. After verification, 80,000 litres of aqueous solution containing 250 to 300 mg of ClO2 (approx. 24 kg ClO2), which spilled onto the plant floor, kept flowing into a pit within the installation’s subsoil. The operator was required to treat these 80,000 litres of aqueous waste and cope with the deliberate loss of electrical power. Drinking water supply nonetheless continued in spite of the degraded operating mode: gravity flow, bleach treatment, reliance on other water extraction points. A dual liquid waste neutralisation approach was implemented for 10 days following the accident, allowing for discharge of treated effluent into the drainage network. Still unable to explain the origin of the leak two weeks later, the equipment involved in this accident plus an identical one at another site were shut down. In light of this accident, it appeared that no drinking water plant operator had ever studied this type of scenario, or at least with such quantities, and moreover the waste treatment firms contacted seemed perplexed by this kind of event. A steering group was supposed to be created from among the various participating bodies in order to determine the origin of this incident as well as issue rules or a memorandum to better manage this type of accident.