At night in March 1998, on a secondary road in Brittany, France, a Ford car crashed into a Peugeot scooter. The car did not stop, and a few kilometers farther on slightly bumped a car, causing it to fall into a ravine. The first accident proved fatal for the scooter driver. The driver of the involved car was quickly identified; he was determined to be drunk. Technical police investigations were conducted on the Ford car that caused the accident. The Motorcraft H4-type bulb (high beams and low beams) from the front left optical set was removed. Along with it, three R10W-type turn signal bulbs and one P21/5W-type bulb of the rear optical set were removed from the scooter. A H4-type bulb was found on the roadside where the first accident occurred. According to the investigators, it belonged to the front optical set of the scooter. All these bulbs were submitted to the forensic laboratory for analysis. The laboratory's initial observations led to complementary police investigations that finally solved the first incompatibility. The different precautions taken by the technical police investigators on the crime scene finally allowed the forensic scientists to establish the functional state of the lightbulbs during the crash. 16 figures and 7 references