During the attack on Clay's warehouse, the thug that Clay captures with his screwdriver doesn't have his shotgun, yet he entered armed. The gun disappears without any explanation (or noise) when Clay subdues him and the thug hold his hands up.
During the final battle, Agent Parker picks up an M4 from a fallen agent in the stairwell. The rifle she picks up has a standard A2 iron sight post. But when she is seen confronting Clay later, her rifle suddenly changes to one she used in previous scenes which has no A2 sight and a different hand guard. Then in the middle of the standoff with Clay, it switches between both rifles a few times.
At around 46 minutes, an FBI tactical unit is stopped from proceeding by the leader of the private security company waving a sheet claiming his team are there on the direct orders of the Governor, and tells the FBI tactical team to leave.
This is absolutely not true in real life. The FBI is a federal agency and is not under any direct state control by any governor of a state.
Regardless of their country of operation, secret service personnel tend not to walk around with large SECRET SERVICE badges on their backs.
In one scene, Clay removes the barrel from a shotgun (on the fly) and stabs the user with it. The barrel of that shotgun must first have the end cap unscrewed and removed before the barrel can be separated from the rest of the gun.
The gear Clay uses at the beginning as a beekeeper is not actual beekeeper gear. He is wearing a fencing jacket and gloves, none of which is sealed, and would have allowed the hornets to get him.
In one scene, Clay removes the slide from a Glock handgun without pulling the trigger. Glocks MUST have the trigger pulled and the slide pulled back slightly before the takedown tabs can be pulled down simultaneously to release the slide.
Clay throws Anzalone through a big pane of glass, which shatters into tiny pieces, showing that this is safety glass. Later, as Anzalone is picked up by law enforcement, he states that he was thrown through a plate glass window. Not only wasn't it a window, it wasn't plate glass either, which breaks into large shards. Plate glass is a figure of speech, the character would not know if it was plate glass or safety glass. More importantly, it is a reference to Beverly Hills Cop (1984).
While Anisette is shooting her Minigun from the Truck Bed, the Ammo Feed belt is hanging and freely dangling meaning there is no ammo box attached to the Minigun.
No explanation for the old tech (especially the CRT computer monitors seen in the film) is ever offered. This is because the story was originally supposed to be set in 2003. As the art department had already started sourcing period hardware, for budgetary reasons it was decided just to keep in what they had already purchased.
FBI Deputy Director Prigg is seen on the phone in one scene. In a scene that follows, he is no longer speaking on the phone, but his image in the mirror shows that he is.
When Adam is talking to Verona in the kitchen having juice and coffee, some crew can can be seen in a reflection on the chrome toaster - the sound man in a white t-shirt, a camera, and someone in the background.
Despite being set in the US, the entire movie is very clearly entirely filmed in the UK. When the SWAT team is entering the Nine Star United building, double-yellow lines are seen painted on the roads (a familiar parking restriction) and a Tesco superstore can be seen in the background. Plus the iconic lift-bridge seen in the film is the Kings Ferry bridge, which connects the Isle of Sheppey to the rest of Kent.
While at the Nine Star United building the agents can be seen climbing multiple flights of stairs to reach the call center while the lights from the police cars are shining in the windows of the call center as if they were on the ground floor.
In the gas station scene ostensibly in Massachusetts, the advertised gas prices are 5723 and 5896. It's unclear where the decimals should be, but four digit prices like that were more common in the U.K. than the U.S. in the movie's period.
Parker states to Wiley that the Minigun fires at 6000 rounds a minute. That is false, the Minigun fires at either 2000 or 4000 rounds a minute which is user selected on the firing controls.