Sullivan and Costello meet in an adult movie theater in Boston's Chinatown. However, an earlier reference to the Patriot Act places this movie no earlier than 2001. By then, the porno houses of Boston's "Combat Zone" were nothing but a memory.
When Frank picks up a Bic lighter at the luncheonette counter after talking to young Collin, you can see the "BIC" in yellow on the plastic part of the lighter. Bic didn't mark their lighters on the plastic until a few years ago.
When Costello enters the convenience store in the beginning of the movie, the cigarettes behind the counter include a blue Marlboro box, which was introduced in the mid-to-late 1990s. When Colin was a boy, Philip Morris USA's flagship Marlboro label had only 2 selections: Regular (a red box) and Lights (a tan box).
When Queenan and Dignam confront Costello it's just the three of them standing there talking. But when Costello says "I've got a date with some angels." The camera goes back to Queenan and you see a fourth person standing there (the FBI guy) whom you've never seen before in the shot.
As Costello walks out of the room where he was hitting Costigan's broken arm with a shoe, you see Costigan fall to the floor behind him. However, in the subsequent brief view from above, it shows Costigan falling to the floor again.
Costigan and French get into a maroon Buick Roadmaster, where Costello is waiting. When Costigan opens the flip-phone, the dashboard in the background is that of a newer Mercedes, with a black interior.
When Costigan is talking to his uncle while his mother is dying in the hospital, a fire extinguisher is on the wall in the hospital hallway. When the scene cuts to the uncle and back to Costigan, the fire extinguisher is gone, but the hook is still visible.
When Costello and Costigan are sitting at the table in the restaurant, discussing who the rat is, a handgun falls to the floor. Costello picks it up and points it at Costigan's face. You can hear him manually cock the hammer, but when the camera angle changes, the hammer is not cocked back.
In the scene where Colin is interviewing Fitzy as his "lawyer", Fitzy is seen talking on the phone but when the camera goes back to Colin you can see Fitzy in the mirror with his hands folded not using the phone. The camera then goes back to Fitzy and he is still on the phone.
After Costello finishes beating Costigan's broken hand on the pool table, he tosses some money down and begins to walk away. As he is walking away, Costigan is seen in the background slipping down from the edge of the table to sit on the floor and nurse his hand. When the camera angle shifts, however, Costigan is still leaning on the side of the table and slides down to the floor a second time.
When Sullivan and Dignam are fighting and arguing, Captain Ellerby holds back Dignam in the first shot, but a millisecond later, he yells "SHUT UP!" in the middle of the room, yet it's impossible to move that fast.
When Sullivan enters the surveillance room monitoring the microprocessor transaction between Costello and the Chinese, Barrigan and Brown are with him. In a following shot, however, Brown can be seen sitting at the far left of a table watching camera monitors with his coat off. In the next scene, he is standing next to Sullivan again and his coat is back on.
When Bill, Costello, and Mr. French are driving towards the Gloucester cocaine warehouse, shots inside the car show them crossing the Zakim bridge northbound. Shots outside the car show them heading towards the bridge southbound.
The taped recording of Costello and Sullivan that Madolyn listens to is from their conversation in the porn theater. However Costello's lines are different on the tape than they were in the theater. One example: While in the theater he says "It won't be me who suffers for it". Whereas on the tape you hear him say "It won't be me who pays for it".
When Costigan and Costello are in the diner with the priests and the nun, Costigan's necklace switches from under his shirt to in front of his shirt and then returns to back under his shirt for the rest of the scene.
After Mr. French breaks and tears off Costigan's cast, the arm is naked with a small amount of bandage in the palm which Costigan then tosses aside. After a brief cut to Mr. French watching as Costello grasps and positions the bare arm, it is again covered in white gauze bandage throughout Costello "beating" it with the boot and the end of the scene.
In the scene at the bar where Costigan and Costello's men are writing down their social security and account numbers for Costello, Costigan corrects Fitzy's spelling of 'Citizens,' and the open end of the envelope is facing to their right. When Fitzy picks up the envelope, however, it switches so that the open end is facing left, then right, then left again in subsequent shots.
When Costello gives Costigan money in the bar to have his arm checked out, he counts out three bills from the wad he's holding in his left hand and drops it on the pool table. In the next frame, the money has shifted to his right hand and there's no money on the table.
When Colin is telling Madylon about how he "Cannot allow you to jeopardize an ongoing investigation...", he lightly puts his hands around her neck and face. When the shot changes and he continues with "...Or your life" his hands are back and he reaches for her face again.
When Costigan and Costello meet at the bar for the very first time, Costigan's cast is bright white, and the top edge is very smooth and straight. Next after Costello calls him to the back room (I ain't the cops, and I ain't askin'), the cast is clearly different, a much duller shade of white, and the top edge of the cast is very frayed.
When Sullivan is checking the street surveillance camera, the envelope is propped up on his coat, which shows tan and blue fabric underneath it. When the angle changes to show just the envelope (and when he picks it up at the end of the scene), it is laying flat and only the tan material of the jacket surrounds it.
As the Providence thugs bodies are being photographed, the initial close-up shot of the body in tan trousers has the feet close together, right calf laying on the left ankle. In the subsequent full view, the legs are moved, with the left leg tucked up more so the right knee is laying over the left calf.
When Madolyn and Sullivan make out in his apartment, just after the phone call he received from Costello, their hands change position between shots multiple times. His hands switch from being on her face to being out of shot and back again, and her hands switch from being on his chest to being out of shot.
When Ellerby briefs the cops in to the surveillance operation he turns around from watching the monitor and stands still. In the next shot he is still doing the turning motion while continuing the sentence.
When Billy goes over to Madolyn's apartment she makes him some tea. While he drinks the tea she says his vulnerability is freaking her out. The front shot of her shows her bending to lean her head on her raised, outstretched arm. Immediately, the shot from the back of her shows her standing upright and arm at her side.
When Queenan is thrown off the 344 Wash building blood is splattered upwards first and then his body hits the ground. Furthermore, a lot of bright red blood is splattered upwards whereas when we get a shot of Queenan's body on the ground there is very little blood visible, and the blood that is visible is dark red.
When Billy leaves Madolyn's office after she refuses to give him a prescription for Valium, she writes the prescription and goes after him without her jacket. In the next scene, she's catching up with him outside the building and wearing her jacket.
When Queenan gets on the train at Park Street station on his way to meet Costigan, Costigan calls him and tells him to get off at the next stop; Queenan responds, confirming that the next stop is South Station. The stop after Park Street is Downtown Crossing, not South Station.
When Costigan and Brown are training at the Massachusetts State Police Academy, the Throgs Neck Bridge, which connects Queens and the Bronx, is in the background. The actual Massachusetts State Police Academy is in New Braintree, a small town near Worcester, with no large suspension bridges nearby.
On the envelope Madolyn receives, Colin's address is shown as 20 Pickering in Boston. Pickering Ave. is in Roxbury, nowhere near Beacon Hill. The ZIP code shown is 02459, which is Newton Centre. In the DVD version, the ZIP code is 02149, which is Everett. Billy's return address is on Conant St. in Boston, but Conant St. is in Revere.
As Queenan leaves the South Station T stop to walk to the South Boston warehouse where he is to meet Costigan, he walks north, away from South Station. However, South Boston can be seen behind South Station, in the opposite direction.
The building addressed "344 Washington" is clearly in the Fort Point Channel neighborhood, just off of Congress street near the Courthouse T stop, looking northwest toward Atlantic Avenue and the Financial District. Fort Point has no Washington St.
The processors in the movie are supposed to be made by a fictitious company called MASS Processor. However, the processors shown have the logo of the Italian chip maker giant ST Microelectronics. The goof is corrected in the DVD release.
When Costigan is taking Oxycontin, the prescription label doesn't list the strength, and the tablet is a medium-sized white oval, like Vicodin or Motrin 800. Oxycontin is a brand name, round pill that comes in a variety of strengths and corresponding colors (white=10mg, orange= 40mg, dark green=80 mg, blue=160mg).
Sullivan is assigned to a detective role immediately upon graduating the police academy. All Massachusetts State Troopers are assigned to field training in uniformed patrol upon graduation and can expect three to five years of patrol work before being considered for any other role or promotion in the agency.
When Billy's phone goes off in the porn theater, it is on vibrate. However, in the alley when he is following Colin, it rings when he receives a text. The event is described in the script by William Monahan: "His phone has fucked up, as they do."
Costigan is smoking in the bar when he orders a cranberry juice before meeting Mr. French. Smoking in bars is illegal in Massachusetts. However, given the number of Costello's men who drink in this bar, it may be that the bartenders are not particularly proactive about enforcing that particular law.
The microprocessors sold to the Chinese government agents are simple 8/16bit MCUs manufactured by STMicroelectronics. They have 6 KBytes RAM and 128 KBytes Flash ROM, and are sold at every electronics store. You might get a rocket in the air with that kind of CPU, but not very accurately. However, someone mentions later that the Chinese got busted with a set of "lamp sockets", and asks what Costello did with the real microprocessors.
When Colin is playing rugby near the start of the film he is playing the position of flanker, but he is wearing the no. 10 shirt, which is the position of fly half. However, some of the players have the same numbers (blue team has 2 number 5s), are out of position (like a 5 and 8 on the flanker spot), both teams have players wearing jerseys in completely unrelated colors (red team has a guy in white) and half of the players are wearing jerseys without numbers on them. Apparently, they didn't care what jerseys they wore.
Madolyn's UMass diploma (visible briefly in a box of her stuff when she's moving into Sullivan's apartment) appears to be a black-and-white photocopy. Real UMass diplomas have an embossed Seal of the Commonwealth of Massachusetts in red, blue, and gold ink raised up from the paper, while Madolyn's is entirely black and looks to just be printed on the surface.
When Billy confronts the guy who robbed the armored truck, he pulls a Walther PPKS 380 caliber semi automatic pistol, he shoots the guy in the knee when you see the gun again the hammer is not cocked which it should be after taking the first shot.
When Colin uses Queenan's phone and redials the last number (Billy's), Queenan's phone displays the called number as 617-869-1469. When Billy calls back from that same phone a couple of minutes later, the calling number shows as 617-177-9528, which doesn't match.
When Frank is running away during the shootout, he gets shot on the left side of his body, and blood hits the wall. In the next shot, you can clearly see him running and no blood nor bullet holes in his shirt or jacket. When he sees Colin, he is taking off his jacket and his bullet wound has switched to the right side of his body.
Near the end of the film, after Costigan is shot, the elevator is blocked. It tries to close but Costigan's body is blocking it, thus moving back inside. However, after Brown is shot, the elevator does not move at all.
When Costigan is shot in the elevator, the pattern of the blood splashed from his brain on the elevator's inside wall is different between shots. In the next shot, blood on the wall is as dry as a bone.
The counterfeit State Police patches on the uniforms and logos
and computer desktops are completely different from the authentic ones on the documentation in the evidence box for Queenan's murder and on the cruisers deployed for the arrest.
After Costigan is shot and killed, and his gun is kicked from his hands, if one listens very, very closely, one can hear the sound of a camera shutter firing. (This indicates that a publicity photo was being taken while the scene was being filmed).
When Costigan is in the elevator after he arrested
and handcuffed Sullivan, Costigan slams against the wall with Sullivan. Sullivan's left hand is supposed to be handcuffed behind his back, but he puts out his hand to catch himself as he hits the wall.
After Costello is shot and killed, his flip-style cell phone rings, even though it is open. In reality, there would be no ring, only a tone in the earpiece, letting the user know there is an incoming call.