In a facility devoted to material surface treatments, at the time of maintenance work, the transfer of a product from permanent storage (20 m³) into a temporary storage cistern caused 3,000 litres of hydrochloric acid to spread over the filling zone. The spilled acid then reached this zone’s retention pond, which was located in the water treatment facility. The gaseous releases engulfed the facility through doors that had remained open and resulted in evacuating the facility’s 25 employees to hospital for observation; they were all placed on 3-day sick leave. The tank truck used for the transfer operation had not been designated to receive corrosive liquids. As soon as the operation got underway, hose fittings on the cistern started leaking. After changing the hoses, the cistern bottom valve failed. An alarm was to be hooked up to the pH meter on the retention pond and a misting device fitted as part of the protective gear to be worn near the filling zone.