A runaway chemical reaction inside a pharmaceutical plant was caused by the excessive heat from a mix of cyclohexane/methylcyclohexane, N-bromosuccinimide and azoisobutyronitrile (AZBN). The reactor’s safety disc ruptured and 400 litres of mix were discharged into the atmosphere. In accordance with the operating protocol, which called for maintaining the mix at a temperature of between 15° and 20°C, the technician heated the reactor with steam at 0.5 bar and then started working on another device. The temperature of the mix reached 56°C within 10 min, at which point the technician stopped heating and stirring the reactor. Since the high pressure level (0.35 bar) had been exceeded, the safety disc ruptured at 0.5 bar shortly thereafter; the temperature was then 70°C. The workshop foreman, who remarked a temperature rise of the reactor up to 84°C two minutes hence, immediately initiated the emergency shutdown routine. This accident was caused by the absence of both temperature regulation and an alarm on the existing sensors, in addition to an inaccurately written operating protocol. Shutting off the stirrer constituted an exacerbating factor by virtue of limiting heat transfer possibilities. The former reactor, which had not been dedicated to this reaction, contained inappropriate safety barriers: a temperature threshold set at 150°C, missing temperature regulator and pressure/temperature alarm triggers. The risks studied had been focused on handling AZBN, thus inducing reaction deviations that had not received adequate attention.