When Dimitrios opens the doors upon entering the Ministry of Marine office, the receptionist is not at her desk nor in the office. But on the following cut, after removing his hat in a gentlemanly gesture while quickly surveying the office the receptionist is all of sudden now standing in front of him.
When Mr.Peters is in Leyden's room and gives him two letters, Leyden holds the letters with his left hand while holding a cigarette with the right. But in the next immediate cut, Leyden is now holding an ashtray with the left hand and extinguishing the cigarette with the right. In addition, he is no longer holding the letters as they are now lying on his lap.
At the Athens Bureau of Records, the clerk proudly tells Peter Lorre that the surname Makropolis is filed in drawer #13 because M is the 13th letter of the alphabet, and the surname Talat is in drawer #20 because T is the 20th letter. Those numbers are correct for the Roman alphabet, but a clerk in Athens should use the Greek alphabet, in which M (mu) is the 12th letter and T (tau) is the 19th letter. However as he clearly says M rather than M and T rather than tau, and given that he has noted their positions in the Roman alphabet, it is clear that he has used the Roman alphabet and not the Greek one, for whatever reason.
In the Paris Metro Station, one of the signs says "Lilas" however in that time period there was no "Lilas" station. There was a "Porte des Lilas" station and a "Mairie des Lilas" station but not a station with the name "Lilas" by itself.
In the Paris Metro Station, one of the signs says "Ballard" however it is spelled wrong. The correct French spelling is "Balard".
In the Paris Metro Station, there are signs saying "Direction". However, at the time when the film was made, the Metro did not have signs with the word "Direction".
In the Paris Metro Station, Professor Leyden does not buy a token. He then proceeds to go through the turnstile without inserting a coin and yet the turnstile is able to turn and in fact does several spins. The reason is that Mr. Peters bought two tokens and put one in for Mr. Leyden when they went through the turnstile.
When Professor Leyden is in the cafe with the journalist inquiring about Dimitrios to the proprietress, at one point the band plays an instrumental version of the song Perfidia. This scene in the story takes place in 1938 as evidenced by the recent invitation to a party sent by Madame Chavez a few days before. The song, however, was published in 1939 and it became a hit in 1940, Therefore, the band is playing a song that didn't exist yet.
In the first shot of the roulette wheel, the audio of the ball does not match the video. For instance, while the ball is still spinning, the telltale clicking sound of the ball landing on a number on the roulette wheel is heard, seconds before the ball is seen landing.
At the end of the film Mr. Peters is arrested and indicates that he will be serving a long prison term for killing Dimitrios. However, under French law he would not have gone to prison since he killed him in self-defense and to also save Professor Leyden.
While Leyden is in the Grodek residence, he asks if he can have a cigarette. Grodek tells him to help himself but Leyden has already taken it. Since Lorre knew that in the script the Grodek character would say yes, he jumped the gun and grabbed one before getting permission. The Leyden character is meek and well-mannered and therefore would not have been so presumptuous.
The name on the sign in the Metro station in Paris reads "Ballard" but Mr. Peters got the name wrong and said "Ballardry".