At a chemical plant, an ammonium carbamate leak occurred in a urea production workshop during the final phase of start-up following 5 weeks of inspection downtime. The carbamate, discharged at a height reaching 60 m, dissociated into carbonic gas and ammonia (with the quantity of pure ammonia discharged estimated at 5 tonnes), thereby forming a cloud that reached the city of Le Havre, causing irritation to many residents. This unit was shut down as an emergency measure and the reaction section drained. The incident was over 40 minutes after alarm activation, with the drainage and product storage steps being completed 6 hours later. The quantity of carbamate not yet vaporized, which fell back to the ground, was rerouted: 4,000 kg of pure nitrogen to the retention basin, and another 2,658 kg discharged into the large maritime canal. No human injuries were reported. The leak occurred on an ammonium carbamate pipe at the level of the synthesis condenser (scrubber) bypass valve. The bypass tap separated from its connection flange on the pipeline, making the tubing tear. During the previous night, 10 hours earlier, a slight leak had been detected by a watchman and was targeted for special monitoring while awaiting the unit shutdown. A defective bypass tap assembly on the pipeline caused this accident. Such taps, adapted to corrosive conditions in the presence of ammonium carbamate, were dedicated to the urea workshop; they were connected to the pipeline by means of flanges screwed onto the tap body and then bolted to the pipeline flange. Disassembly of the damaged bypass revealed that the screwed flange was abnormally positioned on the shoulder of the pipeline neck flange. Ammonium carbamate was thus in contact with the screwed flange, made of carbon steel, initiating its corrosion and then breaking the tap/flange assembly. The operator inspected all valves in the workshop potentially corroded by the carbamate, in addition to studying the possibility of modifying the screwed flanges in order to detect all assembly errors. The plant’s sewer network was reconfigured to complete the system for channelling collected discharges into the retention basin.