Continuity mistake: Costello tells Costigan "your dad had his own code". Then Costigan turns his head and exhales some smoke, but the following frame shows his head in the opposite direction and the smoke is gone.
Continuity mistake: Costigan enters the bar and points at a guy with a leather cap. In the background, a waiter is leaning on the bar, who then stands up and starts to walk. In the following close shot he is back in the previous place, leaning.
Visible crew/equipment: When Costello shows Billy the mutilated hand in a bag, filming crew members standing up are reflected on the wine glass.
Factual error: When Queenan and Costigan are riding the Red Line, Queenan gets on at Park Street station. Costigan tells him, "Get off at the next stop, South Station." South Station is two stops away; the next stop after Park Street is Downtown Crossing. However, Queenan does get off at the next stop, and it is magically South Station.
Continuity mistake: After the rugby game, Sullivan turns to his right to insult the other team. From a different angle he is making a left turn.
Continuity mistake: While Sullivan is rain drenched in military school, when he says "Sir! Yes sir!" his hair style changes in a nano second.
Continuity mistake: When Dignam (Mark Wahlberg) reports the processor theft to the SIU detectives, he says 20 chips were stolen. When Fitzy opens the case at the sale, there are 66.
Revealing mistake: Costigan carries a Walther PPK/S. The hammer on his pistol is down both before and after he shoots the junkie armored car thief. The shot to the leg should have automatically recocked the hammer.
Continuity mistake: When Jack Nicholson approaches a young Matt Damon he has a cigarette dangling from his mouth. When the camera faces Nicholson he has no cigarette. (00:04:55)
Continuity mistake: When a shootout between Sullivan's men and Costello's starts after Queenan's death, one shot initially hits the left glass of the van, then in the next shot the glass is intact, only to be hit again.
Continuity mistake: When French finishes slamming Costigan's arm on the pool table to break off the cast, one small piece remains that Costigan removes, leaving his arm bare. When Costello hits the arm with the shoe, there is again a piece of cast on Costigan's arm and a large piece still attached in the overhead view showing Costigan on the floor.
Visible crew/equipment: When Queenan dies after being thrown out of the building and his body is crushed on the ground in front of Costigan, as he looks at his hand covered in blood, in the shot on the top on the left side of the screen the dolly track of the camera is visible for a second.
Continuity mistake: When Frank asks for "numbers", Madolyn is moving into the apartment. In the next scene, when Costigan and Madolyn "get together" she is shown as moving out, but it seems that Costigan was giving his numbers then leaving the hideout to go get with Madolyn.
Suggested correction: I don't understand why this is a mistake. People often take multiple trips when moving. Especially as she says she is keeping stuff at her old apartment due to having three weeks left on the lease. Also, if Costigan is leaving to meet up with her, why is this a mistake. She is his only friend, what is wrong with him wanting to meet with her?
Continuity mistake: At the end of the scene where Costello smashes Billy's hand, he pulls out a wad of cash with his left hand, peels three bills off with his right hand and throws it on the table, saying "Get your hand taken care of," then in the next shot, has the wad in his right hand and throws that down on the table, and the three bills he just dropped are no longer on the table.
Factual error: When Costigan takes the Dr. Smith-prescribed Oxycontin tablets, the tablets are incorrectly shown as large, white oblong pills with the prescription bottle just labeled 'Oxycontin'. In reality oxycontin tablets are small, round, various colored pills (dependent on mg dose) with the dosage number (in mg) imprinted on one side of the tablet as 10, 15, 20, 30, 40, 60, 80, and 160 (I believe 160 mg tabs are discontinued) and the opposite side imprinted as OC or OP. Also, the dose (in mg) would have been printed on the front bottle label.
Suggested correction: She doesn't give him oxytocin. She gives him antidepressants. Also, medication is not uniform, different companies make medicine in different forms.
Other mistake: When Jimmy Bags reaches for his cigarettes, Billy's gun hits his forehead. But after the cut to French's face, Jimmy Bags is spitting up teeth. (00:43:00)
Continuity mistake: In the restaurant, after Costigan says "School is out," he starts to bring the cigarette to his mouth, but before he smokes, while the action, music and dialogue continues, the shot cuts to him already exhaling the smoke.
Factual error: When Costello throws the cocaine powder and says "Want some coke? Don't stop till you're numb" the pupils in his eyes are constricted. Cocaine, being a stimulant, dilates your pupils. Constricted pupils occurs with opiates - narcotic analgesic.
Suggested correction: It isn't Costello taking the coke. He throws it on the ladies.
Factual error: When the MSP close in on Costello in Sheffield, they come in silently but still have their parking lights on. Pretty useless if you're "sneaking up" to a drug bust. Secondly, patrol officers don't have automatic weapons, as shown in the shootout.
Suggested correction: The parking lights thing isn't so useless. They're so close at the point we see them; anyone on lookout would see them whether they had their parking lights on or not. And it might be that those cars don't have a setting for no lights. My car doesn't; it only has a toggle for parking lights / headlights. No off.
Continuity mistake: When Costello lies dead in the shovel of the excavator, his right arm moves between shots. (First it is up - in the far shot, it is down).
