When Winston and Charles are talking the bar and Winston tells Charles he will help him, he switches from holding a beer to a glass between shots.
The trains from the outside are clearly double-decker trains. When viewed from the inside, they are single deck.
When Charles is cleaning off fingerprints from the car at the lake front he rubs down the drivers window that was shot through some minutes earlier.
Each time the Chicago Metra train pulled into Union Station, the PA announcer kept saying that it was the end of the red line. The CTA (Chicago Transit Authority) uses color coded routes, and the Metra system uses actual named routes (Metra Milwaukee District North). Also, the view of the train entering the city shows it entering from the south, but Charles' briefcase tag says he lives in Wilmette which is north of the city.
When Lucinda and Charles are looking for a hotel the first hotel that Charles asks about is The Renaissance Hotel which is on Wacker Drive. Later when Lucinda asks the taxi driver to stop the cab the cab is again in front of The Renaissance going the opposite direction on Wacker. However, when the cab comes to a stop and they get out of the cab they are not standing out on Wacker Drive, nor in front of the Renaissance.
The Rock Island route on Metra is colored red in the system map and it services as far south as Joliet. Lucinda mentions living in Lake Forest which is north of Chicago and serviced by UP-N (Green) line.
When Dexter asks Schine for help on his baseball quiz, he mispronounces the last name of Red Sox third baseman Rico Petrocelli; there should be no "ch" sound in his name.
When Charles first misses his train, it is raining heavily on the platform. As the camera pulls back, other than on the platform, it is a bright, sunny day with no rain.
After firing his P228 a couple of times, killing the new scam victim, La Roche attempts to shoot Schine. His pistol (which is clearly in battery) goes "click", upon which he is seen inserting a new magazine and then pointing the gun again at Schine, who shoots him before he can get a shot off. What no one one the set seems to have realized is that an empty pistol a) does not go back into battery, b) cannot go "click", and that c) as long as you don't rack the slide or the bolt of a gun that went "click" the probability of it firing on the next trigger pull is pretty exactly zero percent (La Roche's 228, being a SA/DA-gun, would have went "click" again had Vincent Cassel pulled the trigger).
When the police officer starts to tell Schine that he'd better not be in the alley when he comes back around the block, his lip movement doesn't match the words heard.
When Charles is on the train platform watching the train leave, the camera moves behind him. In the reflection from the train's windows, the cameraman can be seen behind Charles.
At the end of the movie, Charlie retrieves this brief case and you are led to believe he gets his money back. The brief case is black, which was the one he gave them with $10K in it. The one with $100K was older and different, it was brown which implies he didn't get the $100K back.