Within a chemical plant, an operator noticed the day before around 8 pm a deformation caused by the accidental pressure drawdown of a 100 m³ storage tank shortly following filling with hot para-nitrophenol. This pressure drop resulted from the intake of a cold substance into the gas cover of the reservoir, due to introduction of a carousel. The accident occurred the next day, with the metallic reservoir roof breaking apart subsequent to a pneumatic rupture during tank pressurization by means of heating and evaporation of water content. The rupture surface extended along the roof/shell weld. Several tens of kg of a yellowish-orange chemical substance projected into the air fell as a spray both into and outside the facility; some 10 kg of the substance surprised motorists travelling in the area, whose vehicles became spotted with colour in very little time. The plant’s medical unit treated 6 individuals suffering from a stinging sensation after washing their cars. The 50 m of pavement contaminated by the falling substance were flushed with large quantities of water. Once collected and treated initially in an external water treatment facility, the washing water was discharged into the site’s sewer system and treated onsite. Total cost of the incident was estimated at € 100,000. The accident had two apparent causes: the rise in tank pressure introduced without any suitable preliminary analysis (i.e. conducted by the assigned operator alone, an informal unwritten operating procedure), and the presence on the liquid seal of a 3-channel valve that one of the broken thrust bearings could position in such a way to isolate the tank from all vents. This position was subsequently eliminated and all valves checked in order to prohibit such a position; moreover, operating procedures were modified to include shutdown of the nitrogen supply network and definition of exceptional operations requiring constitution of a preliminary working group, with a written formalization of conclusions.