In a chemical plant, 520 kg of 2-methyl-1,3-pentadiene (despite being irritating and inflammable, this product is not toxic) in the gaseous phase were released into the atmosphere when a rupture disc (calibrated at 1 bar) broke on a distillation column at atmospheric pressure. Operations personnel stopped the boiler’s heating cycle and cooled the column in accordance with the site’s planned safety procedure. Gas odours could be smelled within a 2-km radius. Fire-fighters and police decided to evacuate the 1,750 students at nearby high and middle schools. Explosiveness measurements conducted on these sites proved negative (perception threshold of the 2-methyl-1,3-pentadiene on the order of 50 ppm). The noncompliance with operating guidelines, which called for opening the “vacuum breaker” valve at the beginning of first condensate recovery, would have caused this accident. This manual valve remaining in the closed position induced an uncontrolled pressure rise in the column and the eventual disc break. The boiler, which contained 3,743 kg of unprocessed methyl-2-pentadiene, had been loaded the previous evening and its heating had been postponed until the next day at the request of the production manager and workshop manager, both of whom sought to adjust the distillation parameters. The distillation pace had thus been tremendously modified without however altering the operating protocol. In order to reduce the probability that such an incident repeats, operating procedures were changed by including a double verification of actions, coupled with conducting a technical study to secure manual operations relative to distillation columns.