Harry Brown makes a reference to the abdominal wound of the gun dealer as probably involving his liver. The liver is in the upper right quadrant of the abdomen, the bullet wound was in the left lower quadrant making this reference inaccurate.
Kenneth (Joe Gilgun) claims the SIG P226R pistol is made from polymer reinforced with fiberglass. This is incorrect as the P226R has a steel frame. However, the Glock 17 he offers for sale does have a polymer frame.
Harry is explaining to Len about his military past and says "When I met my Kath I knew all that stuff must be locked away. I made that decision all those years ago and stuck to it".
Yet, on his mantelpiece, as later noticed by DI Frampton, there is a photo of Harry in his Royal Marines uniform - not all "that stuff" was locked away then.
When Harry is buying the gun there is a girl lying on a the couch. When viewed from Harry's perspective her head is laying flat on the couch, in the shot from across the room her head is propped up against the arm of the couch.
During the riot scene, a Volvo hatchback is torched using a petrol bomb. Later in the sequence the car is seen again, and the fire has gone out. It is possible, although unlikely, that the petrol did not ignite the interior (accelerants in isolation do not burn for long), but the car body shows no oxidation or paint damage, even around the roof.
Although supposedly set in south London, most of the graffiti on the estate has been lifted straight from the New York underground scene by the movies art department, with internationally recognized American Graffiti artists such as 'Kez' and 'Skuf' and 'YKK crew' adorning the supposedly British sink estate.