After becoming soaked while walking around the city in a rain storm in search of Julien, Florence arrives at the bar with her hair dry and still perfectly coiffed, as well as dry clothing and perfect make-up.
Time leaps: When Louis is driving off with Julien's car, the clock on the dashboard reads 5.15, although it is actually about 7.40 p.m. The next moment the clock reads 11.20. After passing by Florence at the cafe, the clock reads 11.50. When Florence is leaving the cafe to start her walk, a clock in the cafe reads 9.30, a few seconds later the clock over a pub reads 2.35. A minute later Julien's watch reads 8.29 p.m. Another minute later the clock in the garage reads 11.25 p.m.
Seen from the outside during the opening telephone call, Tavernier's office has a bare walkway and railing. Seen from the inside during the same telephone call, the walkway outside his office has a shrubbery.
When Julien is leaving the Carala-building, a Vedette is parked in front of the entrance. When Julien is about to drive off from the opposite side of the street, the car is gone. When he is running back to remove the rope, the Vedette is there again.
Carala's office seems to be at the rear side (south-west) of the building. From the rear side windows it's impossible to see Sacré-Coeur. So it's obviously a picture behind the window. It's also unlikely that there are cats climbing up to the 11th floor.
Julien is supposed to climb up at the front side (north-east) of the building. Actually he is climbing at the south-east end, as the building in the background reveals. It's the same one in the background as he is climbing out of the window and the camera is pointing along the building to the south-east end.
When Florence is arriving at the motel, the photos are just being developed - with the lights on brightly! Exposing the prints to light before fixation would make them turn black. By the way: It's not recommended to dive your hands into developer.
Julien gets stuck in the lift while he is going up. When the night guard turns on the power, the lift starts going down. That's a double error. A power failure would cause a complete reset and it requires to push a button to make it start again, as correctly shown when the power is turned on the second time in the afternoon.
The "race" between Louis and Becker is ridiculous. A Chevy could never keep up with a Mercedes 300 SL. At one point, a Citroen 2 CV is driving behind them and keeping up speed. So the speed couldn't be that fast...
After being released from the lift, Julien is running to his car. If not been stolen, it would have been out of petrol after the engine was running the whole night. Furthermore it would have been quite wet inside after the heavy thunderstorm with the roof open.
How could the rope fall down on the street? Being hooked tight to the railing, it's unlikely that wind blew it off, especially as it fell down before the storm. In addition, the building grows wider beneath the railing, so it would have fallen onto the balcony below.
Julien's plan to pretend suicide is foolish. Missing gunpowder residue on Carala's hand would instantly reveal that he was murdered. And since Julien was one of the last who left the building, he would be the one the first to be suspected. The police would find the camera with the shots of him and Florence, and there he goes... By the way: There is not a single drip of blood...very unlikely.
The incriminating photographs on the Minox camera could not have been taken, as that model of camera had no "delay" feature and there certainly would have been no third party present when they were taken.
The goof item below may give away important plot points.
Ventura tells Moreau that Ronet is lucky, in that he would have been executed for the murder of the Germans, but killing his boss would bring a lesser sentence because he had a "reason." This is patently absurd. Killing the German couple would be no worse than second-degree murder (and he could easily claim self-defense, as the German also had a gun), but Ronet planned his boss' killing, which would certainly be first-degree murder and thus carry a far harsher sentence.