Mannix's much-loved wristwatch is often shown and is octagon-shaped in close-ups, but in long shots, the actor is wearing a rectangle-shaped watch.
In Baird Whitlock's final speech, his ruffled collar is alternately tucked into his armor from one angle and standing straight up from other angles.
During the "No Dames" dance number, there's a drunken sailor passed out at the bar. When Burt Gurney does a slide along the bar and other dance moves, the drunken sailor is no longer there. Later, when Gurney leaves the bar, the sailor is back at his spot, still passed out.
When CC turns to see Eddie entering the cutting room, her cigarette disappears and then reappears in successive shots.
When Burt Gurney puts a bar rag on the bartender's head and then slides along the bar, the bartender can be seen starting to remove the bar rag. In the next shot, a close-up of the bartender, the bar rag is still on his head with him not trying to remove it.
When Mannix is viewing the rushes (or dailies), they are being shown in color. Rushes would have been printed on inexpensive black-and-white stock as they were used only for cursory approval purposes.
When the scarf gets stuck in the editing deck, stopping the film, the lamp melts the frame on the screen. The lamps in editing decks are much lower intensity than those used in projectors and do not produce the heat necessary to melt stuck film.
When Eddie is having lunch with the Lockheed headhunter, he is shown a photo of a nuclear test. He's told it's a Hydrogen Bomb test in Bikini Atoll. It was an Atomic Bomb test at Bikini Atoll in July 1946; the hydrogen bomb hadn't been invented yet, much less tested, in 1951. The first hydrogen bomb test (Ivy Mike) took place on Enewetak Atoll, not Bikini Atoll, on November 1, 1952.
In the editing suite, the female editor is seen smoking as she operates the editing machine. Since the film is set in the early 50s, the film would have been nitrate which was highly inflammable. Potentially, the whole studio could have burnt down.
In 1951, film production was transitioning away from nitrate to safety (non-flammable) film stock. The fact that only one frame is burns indicates that the editor was working with safety film. Had it been nitrate, the entire reel would probably have caught fire.
The Soviet submarine is shown beginning to submerge before the hatch is closed. No submarine commander would order submergence until he could verify that the boat was sealed.
When the cucumber sandwiches are being prepared in the kitchen for the communist study group, they are shown as having the crusts cut off in thin slices. When they are served to Baird Whitlock, they are cut into squares, without crusts.
Near the beginning of the depiction-of-God discussion, the rabbi says "for we Jews", rather than the grammatically correct "for us Jews" (it is the prepositional use, not as a conjunction), and should know better.
When Eddie's secretary tells Eddie that he has a call on line 2, Eddie picks up the telephone handset but does not press the line 2 button. The last call Eddie had on that telephone was on line 1, so he still would have been using line 1 on that line 2 call.
At around 22:30, a second or two after the car passes, the North 101/ Hwy 1 pole shakes.
When EDDIE MANNIX is reviewing film in the editing bay. he is looking at scenes from MERRILY WE DANCE. However, at the start of the movie, we are told that this is also the first day of shooting MERRILY WE DANCE. Not only is the sequence MANNIX is watching from MERRILY edited, but it has opening titles. (Titles would have been added only when the filming was completed.) Also, the scenes we are seeing in the editing room are not the scenes HOBE was shooting with LAURENCE LAURENTZ. When were the edited scenes that MANNIX is watching shot? When were the processed? It is literally less than a couple hours since the scenes were filmed.
At 20:45, the scene where Scarlett Johansson's character is rising up from the water is obviously shown backwards as she's perfectly dry. Also, seconds later, when she's being taken her costume off, she shows no evidence of water in her body.
Hobie Doyle's Western movie called "Lazy Ol' Moon" has a copyright date of 1951, but claims to have been made in VistaVision, which has yet to be invented. In 1954 Paramount Pictures engineers created VistaVision which is a higher resolution widescreen variant of the 35mm motion picture film format, which allowed the same cinema print of the film to be shown in either the normal Academy ratio of 1:37 or could be soft matted into widescreen aspect ratios between 1.66 to 2.00:1.
When Gurney is about to leave on the Soviet sub, the head writer tells him to take the ransom money as their "modest gift to the Comintern." The Comintern had been dissolved in 1943, something that party members would have known.
When Hobie is picking up Carlotta for their date, a car turning right onto the avenue down the hill is briefly visible. It is white and a modern vehicle, a small SUV make.
The letter from Lockheed that Eddie Mannix reads is from "Human Resources." At the time of the events in the movie (and for several decades afterward), that department would be called Personnel.
A modern-day skyscraper is visible behind some trees in the first scene where Thora Thacker speaks with Eddie Mannix in the studio courtyard. The skyscraper is "The Tower" at 3900 W Alameda Ave, Burbank, CA 91505, constructed in 1989.
The beach-side house where Baird is being held for ransom has a close-up view of Anacapa and Santa Cruz Islands. The film alludes that it is in Malibu, but that is too far from these Channel Islands to have such a view.
The voice-over narration for the film-within-a-film of Hail Caesar! says that the stomp of the Roman army's sandals can be heard from the Iberian Peninsula in the west "through the halls of the great library of Alexandria in the east!" However Palestine, where much of the story takes place, is located east of Alexandria and was also under Roman rule.
The driveway for the beach-side house is shown as being just east of Mugu Rock, currently part of Point Mugu State Park. The bay there is far broader than the narrow bay depicted in the film. In addition, the profile of Mugu Rock is far pointier than in reality; obviously a matte painting.
When Baird Whitlock is kidnapped, the communists threaten to reveal his secret about the "On Wings As Eagles" scandal if he names his kidnappers. It therefore made no sense for Burt Gurney to leak the story to Thora Thacker as Baird would then have nothing to lose by revealing who took him.
The harpist is spiking Baird Whitlock's chalice in plain sight. Although the cast and crew are waiting in their places doing nothing, no one notices.