Glycol was accidentally found in the drinking water network at a plastic technical parts manufacturing plant. The presses and moulds were initially cooled by a single circuit. Then modifications were made to improve temperature adjustments by separating all of the cooling circuits. Since then, each machine has been equipped with both an external general glycol water cooling circuit and a mains water-operated cooling circuit specific to each press. Only 2 valves, whose control devices have been removed, separate the circuits. The cooling machine’s start-up instructions, displayed on the work station, state that these valves must not be operated. However, a valve reconditioning campaign had been carried out, including on those that should have remained locked. At the time of the incident, a technician was in charge of topping up the mains water on the internal circuit of press no. 15. Therefore, he would have opened the tap water intake and the separation valves between the 2 circuits. As the water in the glycol circuit water was pressurised to 4 bar and the water in the mains system was at 3 bar, with no check valve, the glycol water flowed into the mains system. The valves remained open for 2 hours allowing 18 l of glycol to escape into the plant and mains water networks. As the company’s taps were producing foamy water, the maintenance manager rapidly identified the problem and closed the network’s water intake. Three people were hospitalised. The water department drained the company’s network and part of the municipal network. A check valve was installed on the tap water inlet. A ban on using polluted tap water was displayed on all of the site’s water points. Following this accident, the inspection authorities for classified facilities recommended the issuing of a formal notification order obliging the operator to install a backflow preventer on its installation and to submit an accident report stating the causes of the accident, the effects on people and the environmental impacts and the measures taken or considered in order to reduce the likelihood of a recurrence.