Dimethyl sulphate (DMS) began leaking around 11 am at a chemical plant as the product was being loaded. The connection between the DMS container and the loading station consisted of disassembling the solid flanges, replacing the joint by a new part and reconnecting the container flanges to the unit’s pipe flanges. After initiating DMS loading in the control room, the field operator climbed down to inspect the container and, at that point, identified a leak on the flange connecting the container to the loading pipeline. He sounded the siren and the emergency light before pressing the emergency stop button. The next day, the plant operator concluded that the leak had been caused by poor clamping of the loading flange while the container was connected to the loading station. Moreover, the safety automated system was not activated because the pushbutton had not been held down long enough for its cycle length (1/10th of a second). All emergency stop pushbuttons were replaced by locking buttons throughout the site.