The script originally had no back story about Robert, so Denzel Washington contributed much to the character's background and back story, including McCall having obsessive-compulsive disorder (OCD). In addition to his daily physical and fight training before filming, Washington met and interviewed several real-life OCD sufferers in order to gain insights on how to play that disorder correctly.

According to the director, during the research for the role as an underage escort, Chlo? Grace Moretz interviewed real-life prostitutes. She met an escort who initially told her to gain weight, because in that profession in order to take care of different men, escorts have to have some flesh. Moretz, who was a minor at the time of filming, changed her diet and gained weight for the role.

Slavi refers to Robert as "dedushka" when they meet at the restaurant. This is Russian for "grandfather."

Chlo? Grace Moretz fought for the role of Teri when she found out it was written for someone of 24. She presented herself so well, Antoine Fuqua immediately made the change.

Teri was originally written for an older actress, but Chlo? Grace Moretz impressed director Antoine Fuqua so much, he made the role younger. She was 17 years old when the film was released.

Continuity

At the beginning of the movie, while getting ready for work, Bob dresses in slacks and a button-up shirt. He is wearing the same outfit while at work, but on his way home, when he gets off the bus, he is wearing blue jeans. Once he arrives at his apartment, he is wearing the slacks and button-up again.

During the final fight, Ralphie turns on all the lights in the store and yet when McCall faces Teddy, only few lights are on.

Robert and Teri walk down the street in East Boston and then stop and talk. A woman and child walk by them twice in the same direction.

When Bob is closing his wound using boiled honey you see an empty bottle of honey by the stove. Then you see a full bottle of room temperature honey in the bathroom with him. He keeps a lot of honey on hand.

At Homemart after robbing the cashier of till money & her ring Mr. McCall follows the robber to his car the robbers partner revs the car engine the robber gets in the car & he & his partner drive off when they show the rear car window there is only 1 robber in the car, The driver.



Factual errors

The quote shown at the start of the movie, "The two most important days of your life are the day you're born and the day you find out why." is attributed to Mark Twain whereas the source of this quote is actually unknown.

In the diner scene with the Russian in the power/construction outfit, sticking a dinner knife in the outlet would not have made contact, much less short the main breaker even if it did make contact with only one terminal side.

When Robert and Brian are playing chess on the back porch, both of Brian's white bishops are on white squares. That is impossible.

After getting shot in the diner Robert heats honey to boiling and then used it to treat his wound. Presumably this is to prevent infection. Honey heated to boiling would lose its antibacterial properties.

At one point, Robert is riding a Red Line train, but the interior is not correct. Red Line trains in Boston have much wider cars, and Robert is shown in a train that more like the narrower Orange Line train he was on earlier in the film (probably a cut of the same footage).



Incorrectly regarded as goofs

When Masters is told to 'Call it in' while handcuffed to a pipe at the money counting warehouse, he calls the police dispatcher and reports that he is "10-13 at the rear entrance". The 10-13 call sign is for reporting weather conditions and not police status at a crime scene. Perhaps 10-78 (Need Assistance), 10-31 (Crime in Process) or 10-23 (On Scene) would have been more appropriate. Police 10 codes vary from city to city. In Boston, 10-13 does mean "Officer needs help".

In his flashback, Bob wears his wedding ring on his right hand instead of his left. A wedding ring on the right hand signifies a widower.

(at around 31 mins) Flipped shot. A brief transition scene shows a Boston "T" Red Line subway car traveling from left to right. All the writing on the subway car is reversed. A flipped shot is done in post-production (the camera doesn't film backwards) and is intentional. They are clearly intended by the filmmakers, for humorous purposes or ?otherwise?. Since a goof is an ?unintentional? error, by definition flipped shots are not Goofs.



Revealing mistakes

Robert watches the men in the apartment using covert, hidden cameras. Both the camera in the clock and the camera in the toy train have small red LEDs to indicate they're on. Covert cameras don't do that for obvious reasons. And no, they're not infrared lights for low-light situations.

When Masters checks his revolver after receiving a call from Teddy, all the primers (center part on the back of a round where the firing pin must strike to set it off) have all been dented. This means the bullets have been fired, and nothing would happen if the primers were struck again. The film makers probably didn't think anybody would notice during the half-second the image was onscreen.

As McCall walks behind Jenny at the register when the Home Mart is being robbed, the screen of the small customer-facing register display is blank with 4 blue motion tracking marks in the corners where they forgot to composite in a fake screen.



Miscellaneous

During the very loud hand-to-hand fight in the store the others would have had plenty of time to run to that location and end things.

The blade on a standard pole saw is very flexible and would not be rigid enough to be used as a spear.

When Robert is sitting in the diner with Teri, he is a tea drinker yet Jake comes up and pours him a cup of coffee. If you watch closely Robert goes to put his tea bag in the cup, then realizing the error puts the tea bag down on the napkin.

When McCall anonymously contacts FBI Agent Mosley in charge of the warehouse bust & sends the Agent the Zip File containing all the corruption between the Russians & Boston PD, McCall tells the Agent to check his personal email, but when the email is being shown as "sent!", the address shows "sa.mosley@fbi.gov", that would not be his personal email but rather his professional email. [NOTE: In many organisations, including some government agencies, individuals have access to a group email, so anybody within a team can respond when the responsible worker is not available, or for team tasks, and an individual email for their personal attention. While this is a professional email address, it would also be considered a 'personal' email.]

After fight with Russian mobs, around 30 minutes into the film, McCall stopwatch ends with different number of seconds (00:28:24) versus the same scene in the trailer (00:19:24). Yet, none of these correctly catches the duration of the fighting scene.



Audio/visual unsynchronized

During the final "battle" in the DIY warehouse depot, one of the enemy activates night vision equipment. In reality, it does not make a high pitched sound when turned on.



Errors in geography

The power sockets next to the mirror in the Moscow bathroom are UK style, not the Russian Type.

The whole "Moscow Mansion" is not in Moscow. First, the identification numbers on the security cars outside are clearly of the European standard with the blue field at the left side. Second, the windows are opening like in UK, or in USA - upside down.



Plot holes

When McCall cuffs Frank in his car and forces him to speak by gassing him, Frank should have no problems opening the door or lowering the window from inside the car, having one hand free.

The fight with the broken glass is loud and takes place over the span to two minutes. The other Russians would have heard this and arrived in time to kill McCall.

Ralphie needs to be a certain weight to qualify for security guard. He reveals he makes the weight at 247 pounds. There is no way he is 247 pounds, considering the height and and weight McCall is and they are about the same height.



Character error

Teddy sees surveillance camera footage McCall and the girl leaving, but then later tells his cohorts the video shows Teddy enter but never leave.