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.
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.
When Julien throws the grappling hook up, it catches on the second rail. When he gets to the top, it's on the third. It continues to alternate between the two.
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 Florence arrives at the motel, the photos are just being developed - with the lights on! Exposing the prints to light before fixation would make them turn black. By the way, it's not recommended to put your hands into developer.
Although the Mercedes has a D-badge for Germany, the registration number R513-772 is Austrian.
The "race" between Louis and Bencker 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.
Florence should have seen that someone else was driving Tavernier's car, as the POV shot makes clear. However, it is clear in her POV shot that she can only see who's in the passenger seat.
Louis and Véronique ditch Bencker's car at the bridge, then run to her place. When Florence is in the phone booth to the police, she sees him leaving on the scooter he stole. How and when did he retrieve the scooter? Actually, he had left the scooter at her place before going to work the next day and killing his boss.
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 someone to push a button to make it start again; this was correctly shown when the power is turned on the second time in the afternoon.
When Julien is hanging from the cable and the elevator suddenly starts moving down the shaft, there is a shot where the walls around him aren't moving as they should.
The Benckers speak to each other in French when they are alone instead of in their native German.
When Louis and Véronique are passing by Florence on Boulevard Haussmann, it's not Louis who is driving.
Being an executive at the company, Tavernier would have his own parking space instead of having to park on the street.
As Julien walks along the balcony before throwing the rope, a crew member's reflection is visible in a window.
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.
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.
When Louis can't start the Mercedes, Horst Bencker reveals that the car won't start because he's taken first gear out. However, with no further changes, Louis is able to drive off five minutes later.
Julien climbing up and down the balcony would be seen from the opposite buildings, which are nearly as high as the Carala building.
Julien's plan to pretend Carala committed suicide is foolish. Missing gunpowder residue on Carala's hand would reveal he was murdered; since Julien was one of the last who left the building, he would be one of 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 was not a single drip of blood, which would be very unlikely.
It's unfathomable that the florist would give Florence or any stranger who wasn't law enforcement Véronique's address.
Louis nor Véronique ask Florence the obvious questions except who she is, but she never answers that question, and he doesn't ask again.