In a pyrotechnic facility, a deflagration occurred during the pelleting of black powder. A technician, after verifying the hopper feed line, turned on the pelleting machine. The blast took place just a few seconds following start-up of the compression cycle. This incident was the fifth of its kind on the particular machine. The cause could not be determined with certainty. The hypothesis of static electricity seems to have been rejected. The presence of a foreign object or powder accumulation in the blind hole of the punches constituted the most likely hypothesis. The pelleting machine and adjacent devices were damaged, the building roof and facade were destroyed, and 30 kg of powder was lost. The technician suffered a slight concussion. The operator adopted a number of equipment-related measures to avoid the presence of foreign objects in the black powder. The machine configuration was modified in order to limit the mass of active material capable of entering into reaction.