The cross tattoo on Jimmy's back changes size from small to large.
When Jimmy pulls out his knife, his glove disappears then reappears in subsequent shots.
When Dave comes out onto the porch to smoke he is startled by Jimmy and drops his match before lighting his cigarette. Cut back to Dave and the cigarette is lit.
When Jimmy Markum is drinking whiskey with Celeste Boyle, the cap of the bottle is alternately on/off between shots.
(at around 2 mins) When the boys are writing their names in the concrete, Jimmy starts his name at the top of the slab. When Sean scrawls his name, Jimmy's name has moved to below center of the slab.
As kids, Jimmy was the freckled red-haired boy and Sean was dark haired Italian style. When they grow up Jimmy is the one with dark hair Italian style and Sean is much more likely to been the red-haired boy.
When Theo comes to Katie's Wake, he hands Jimmy a case of Miller High Life. When they go put it in the cooler, it's Miller Lite.
When Jimmy and the monument maker are conversing, the monument maker says one option is a "red marble" stone. When he points to the stone he is talking about, he is pointing to a stone that is obviously granite, not marble.
When looking for the missing girl, all the Troopers are wearing their winter uniforms although the trees are all green indicating summer or late spring. Troopers change to summer uniforms on April 15th when as anyone from Massachusetts will tell you, the trees are still bare.
At around 5 minutes in, as the young Dave Boyle is being driven away by the pedophiles, there's a cut back to the young Jimmy and Sean standing in the street. In the background to the right, in plain view, is a modern era police car guarding the filming location.
In the opening scene that takes place in the 1970s all of the cars have front plates. Massachusetts did not have front plates until 1988.
The type of license plate on the car driven by the "cops" in the opening scene, which takes place in 1974, was not used in Massachusetts until the mid-1990s.
In early street scenes, set in the '70s, there's a "no trucking" sign on a pole that employs the red circle with the line through it, 1980s style.
When Jimmy is pouring himself a drink, the sound of pouring liquid continues although the level of the drink in his glass ceases to rise.
When Dave is writing his name in the cement, he can be heard writing much more than two letters. Although he supposedly writes only DA, more strokes of the stick are heard.
In the liquor store scene the reflection of a white-shirted cameraman is clearly seen on the glass door of the large beer chest.
(at around 5 mins) When the two "cops" take the kid away at the beginning of the movie, you can see the camera lens and the hand of the operator as they are about to pull away.
When Markum and family are leaving the first communion and they see police cars fly past the church, the shadow of the camera falls across Markum's back as it moves around him.
The Cantab Lounge is a folk and bluegrass club in Cambridge, between Harvard and MIT. It is highly unlikely that Boston police would congregate there on any night, much less grab a drink there.
In the closing credits, the producers thank the City of Canton. Canton is in fact, a town in Norfolk county, MA and not a city.
(at around 28 mins) When Jimmy notices his daughter's car for the first time, a boom mic is revealed through the trees.
(at around 15 mins) Dave is seen at a bar watching Red Sox game on TV. The announcer is Jerry Trupiano, who was on the Red Sox radio network. Trupiano never did games on TV.