On the site of a motor vehicle gearbox manufacturing plant, a machining cutting oil drip collection tank overflowed on the Saturday of the Easter weekend. The cutting oil soaked into and then passed through a non-watertight wall and flowed into a drain in the establishment’s stormwater network. The detection of hydrocarbons in this network activated the automatic closure valve. However, oil had already flowed further downstream and had spilled into the River CHENEAU. On the following Tuesday, the water board noted pollution of the stream and alerted fire-fighters who installed 2 booms across the river. Research carried out by employees of this department identified who was responsible for this discharge. The inspection authorities for classified facilities were notified on the same day and carried out an investigation. Equipment failures and organisational anomalies had caused the accident: malfunction of the upper level monitoring device on the oil tank, excessive delay in closing the shut-off valve, spillage into the public network not detected by the operator, and insufficient internal communication (the plant’s environmental department only became aware of the incident following the inspections conducted by the water board and the inspection authorities for classified facilities). The operator repaired the upper level monitoring device and paid the costs of flushing out the network and disposing of the hydrocarbons recovered. The inspection authorities recommend that the Prefect issue an order for additional requirements.