The presence of flammable gases and vapours in wastewater has been a well-known danger for decades, also due to the many accidents of the past, both in purification plants and in process plants. In hot work, in particular, it is essential to implement all the necessary measures (e.g. emptying, reclamation, insulation) to prevent sparks from igniting vapours and causing explosions and fires. Although these things are all well known, accidents of this type continue to happen even in the Seveso sites, moreover subject to strict controls. This article talks about a destructive accident occurred during hot work at a wastewater treatment plant in an upper-tier Seveso chemical plant. The paper highlights the lessons learned, in both organizational and plant terms, and the related return of experience for technical evaluation and control activities, starting from the consideration that the accident was originated during a relatively simple plant modification activity carried out by a third-party company.