Throughout the film, which takes place during a short time interval, the deciduous trees switch randomly back and forth between full summer green leaf, fall colors, and without any leaves.
When Spenser is thrown out of the bar the street outside the bar is full of parked cars, including directly outside the door. When he goes to store across the street and looks at the monitor from their camera it shows the door to the bar but with a near empty street out front. Then when watching the camera later (somehow from a different angle) the parked cars are back again.
After nearly getting his nose broken in the Slainte bar bathroom fight, it's in perfect condition when he gets thrown out and stands up.
When Spenser finds the camera monitors in the store he sees the camera that is pointed at the bar. The shot is a medium shot of the front door from down the street. When he asks the store clerk to show him some footage the shot changes to a shot from directly above the door of the bar. Later when he and Hawk are watching footage, the shot is now wide enough to see the parking lot beside the bar.
Around minute 49 Hawk slashes the yellow Corvette's tires. But only a few moments later, all tires are in perfect condition again.
The Corvette shown is not a Z06; it is a Stingray.
The dog that attacks Spencer, Lego, is referred to by her owner as a girl ("good girl, come here") yet Spencer says "you need to neuter that mutt". Female dogs are spayed, not neutered.
After Spenser approaches the FBI surveillance car, the agent with the computer just starts typing Spenser's name into his database search without knowing who he is. He even acknowledges he doesn't know him by being shocked to learn of his past. However it is obvious that the other agent recognised him and told the first agent his name.
Start sequence, and later in movie, when Spenser as a cop is seen approaching Boydin's home, the cop car seems to say "Boston Police" on the side, but "Bobton Police" on the tail end of the car. However, close inspection shows that it is BOSTON POLICE with the S of Boston lacking the vertical left hand side to be a B.
When checking the security tapes, the Corvette appears to be green, but is yellow in the rest of the movie. The tapes are tinted blue, which would make a yellow car appear to be green.
During the fight in the bar toilet, Macklin and Spenser both trade punches to their respective faces, yet afterwards outside on the footpath, Spenser's face is relatively intact but Macklin looks to be sporting a busted nose and split lip.
In the news clippings at the end, there is a typo in the lead that announces local heroes busting dirty cops, namely an extra 'and': "a former police officer and an and amateur boxer."
In the beginning, when Driscoll welcomes Spencer back from prison in front of Spencer's home, Driscoll's toothpick is inside his mouth, but in next scene, it is not. Then it is in his mouth again after he embraces Spenser.