While Dottie talks to the press, she uses the word "visual" when she means "vigil." This was because her lines were being fed to her by director Ben Affleck and she misheard this particular word. Affleck noticed her mistake but thought it sounded natural, and thus left it alone.
Det. Bressant talks about rolling into a Columbia Point crack house in 1996 to make a bust. Columbia Point was notorious for drugs and crime but was knocked down in 1989. A new, largely market-rate development called Harbor Point was built over it in 1990.
When Det Poole is shot outside of the crack house, the window from which he is shot is located to his right. When he is shot, he clutches his hand to the left side of his neck. There are no visible wounds to the right side of his neck, where they should be.
When a police officer compliments Patrick on the way he handle the events at the quarry, the officer says "...the other night..", but it's been previously established that this scene is occurring two months after Amanda's disappearance.
When Patrick and Angela meet Detectives Brassant and Poole for the first time in the diner, they are going over the rap sheet for Roberta Trent. She had some charges in Lawrence and Lowell where the ZIP codes were listed 02114, which is the ZIP code for Boston. Lawrence and Lowell's ZIP codes start with 01 (i.e 01843)
Bressant says he and his partner went to a "crack den" in the Columbia Point (Housing Project) in 1995. In reality, by 1995, Columbia Point had for roughly 5 years, been rehabbed as "Harbor Point", a high-end mixed-income development. The kind of crack den that Bressant says he broke into had pretty much been closed down.
When Detectives Bressant and Poole are preparing to raid the house, they load a shotgun with rounds from a blue plastic box. The rounds in the box all have indented primers which would make the ammo useless.
The goof items below may give away important plot points.
At the end of the film Helene states a man from Rhode Island is driving down to see her for a date. This geographically incorrect as Dorchester is north of Rhode Island hence he would be driving up not down.
In Nick's funeral procession the boot in the horse's stirrup is backwards, and there is no leg in the boot. This is done on purpose; a riderless horse or caparisoned horse is part of a state funeral. This might still be a goof, though, because the honor is usually accorded to military commanders, ranked colonel and above, rather than police officers.
Patrick is in Helene's home staring at the framed Boston Herald's front page titled "Amanda Comes Home". At first glimpse of the Herald, the Boston Red Sox logo is on the upper right portion of the paper. Later it is seen with the Boston Bruins logo.
During Nick's funeral service, the DVD subtitle reads "Latin being spoken". This was a mistake in two ways. 1) The language being spoken is actually Greek. The funeral service appears to be a Greek Orthodox service. 2) Probably the DVD technician wrongly assumed it to be a Roman Catholic service. But even so, the Catholic church does not regularly use Latin, not since Vatican II Council in the early 1960s.