Jean-Paul Belmondo's character, a Paris police inspector, wears a new Rolex Daytona watch, which at the time would have cost about $2000 US. This is far more than any honest Paris police inspector could have afforded. Today, in good condition, the watch sells for more than $25,000.
Writer/director Henri Verneuil was inspired in this story by what once told him Françoise Fabian, herself victim of a peeping tom maniac who also harassed her by phone.
For the helicopter scene, the (the Intervention Group of the National Gendarmerie, only two years old at the time) authorized the use of a real Alouette (the helicopter), on the condition that the raid was done on an unoccupied building (all residents had been evacuated beforehand) and real GIGN agents were cast as the other officers that followed Jean-Paul Belmondo into the window. For some subjective shots in this sequence, Belmondo himself was operating the camera.
For the metro scene, the RATP (the Parisian Autonomous Transit Network) allowed the crew to film the chase scene from midnight till 5am (when the metro is closed), for three weeks! At the end, the metro driver said to Belmondo, "I wouldn't have done this for a million Francs (around 200,000 modern Euros)!" To which, Belmondo replied, "neither did I!"