During the metro chase, the train driver is obviously alone in his cabin.
However, when the train passes the overground bridge three people in the cabin are easily visible.
When Belmondo chases the bank robber into the metro, he's clearly packing a .357 or .44 Magnum. However when they finally have the shoot out on the train, his gun has changed to a snubnosed .38.
When Letellier's driver is shot during the car chase after the bank hold-up, he sinks on the wheel and blows the horn. That's impossible, because the Peugeot 504 has no horn-button or -ring on the wheel. It's blown by pulling a lever.
When Minos visits the police station in order to talk to Letellier, two healthy eyes can be seen behind his sunglasses.
The chase in the subway (Express train) starts at the station Auber, in front of the Galeries Lafayette store. Minos and Letellier are in a train that leaved Auber Station and get off at the next station (that should be Charles-de-Gaulle-Etoile), but we see commissioner Letellier in the Auber Station corridor. At the same moment, Minos (who should be in the same station that Letellier) is on the subway platform of Charles-de-Gaulle-Etoile Station.
When commissioner Letellier is chasing Minos in the subway, and is running on a train roof in stations, we see the same station twice.
During the chase of Minos on the roofs, Letelier and Minos are on the Galeries Lafayette store roof. When Letellier lost Minos, he leaves a building that is supposed to be closed to the store? In reality, he couldn't arrive in this street by the roof because there's a street between the two buildings.