During the final battle between the gangs and Kersey, a small two-story wooden structure is seen burning out of control on one of the neighborhood streets. While firemen fight the flames, the facade collapses into the street and the whole structure is consumed in fire and smoke. In several of the following battle scenes however, the structure is seen unscathed and totally intact.
Towards the end of the film a gang is pursuing Paul rivals down a street lined with Stanton Staveley concrete lamp posts. These are common in the UK, which gives away the location as South London. These types of posts are not found in NYC and are extremely rare in the USA.
A LAWS rocket is an antitank rocket housed in a disposable launch tube. It is not reloadable as shown. Nor does it have a pistol grip. The launcher has a simple visual sight and a button on the top of the tube to fire the weapon.
When Kathryn's car rolls downhill and slams into another car, both vehicles spontaneously explode. This is extremely unlikely at these low speeds (both cars were going at around 30 mph). Thousands of people are killed in car crashes every year, but at much higher speeds and without any explosions. Although gasoline is very flammable, it needs to spill outside and a flame to ignite. At 30 mph crashing cars do not produce enough heat for ignition, especially when gas tanks are in the rear of vehicles and even if gas is spilled, it ends up on the ground, not on top the hot engine block. Since Kathryn regained consciousness before the crash, she would've had plenty of time to escape.
The Browning 1919A4 machine gun is recoil operated. Without a solid mount--pintle, tripod, bipod on the ground--it is guaranteed to misfire within a few rounds and require cycling the action again. Long bursts as shown are impossible when hand held.
While Kersey is having dinner with the Kaprovs in their first-floor apartment, he goes out and shoots the two car thieves. Yet when he returns from outside, he's walking down from upstairs. He should have been climbing up from the ground level.
Charlie (Kersey's friend who was killed) and Bennett (who helps Kersey) are supposed to be neighbors living in the same apartment building. However, their apartments are as different as they can be. Slum buildings often have identical flats. And even if some of them do differ from the others, Charlie's and Bennett's apartments are wildly different. Bennett's ceiling is almost twice as high as Charlie's, and his living room is also more than twice the size of Charlie's.
When Fraker incapacitates Kathryn (Deborah Raffin) in her car, his thug, without any key, switches the transmission to neutral so the car will roll downhill. In automatic transmission, it is very complicated to put in neutral without a key. It involves crawling underneath the car with the wheels blocked. If done without blocking wheels on a hill, the car will immediately starts rolling down and crush whoever is tampering with it underneath.
(around 1:14.05) Kersey starts firing the .30 cal machine gun....the ammunition belt that you can see is about 3 feet long and Rodriguez is still holding the box...he fires a short burst which takes out the car then Rodriguez claims they are out of ammo.
At around 00:31, when the thugs just killed a rival gang member, the mob leave cheering and we clearly see that someone in the forefront picks up a toilet plunger (???) How does a stick with a large rubber end can be used in criminal assault ? It can give a nasty scare to a germophobe, but other than that its ability to cause harm is severely limited.
When Kersey finds boot prints in the apartment, they are not the prints of someone who was walking. Rather, it looks like the prints were made by someone jumping repeatedly with both feet at the same time, like playing hopscotch. Furthermore, it looks like the person has walked on fresh black paint before breaking in.
(Incorrectly regarded as goof) Bennett shows Kersey an old US Browning .30 lMG in his cupboard. When the punks attack him, he suddenly takes out a German MG-42. Bennett did mention that he had 2 machine guns.
The police chief explicitly states that handguns are illegal in New York City early in the film. Yet in the climax, many concerned citizens have their own handguns. Where did they get them from, if ownership of them is illegal in the city?
When Paul fires his Wildey magnum at the thugs on the outside apartment stairwell, as he fires the gun, the slide pulls back, But when he fires again, the slide doesn't go forward to reload for the fired shot.