(at around 5 mins) As the young Dave Boyle is being driven away by the pedophiles, we 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.
When Sean and his partner are in the car talking about how the gun used in the liquor store hold-up by Just Ray relates to the murder, Sean is seen loosening his tie and unbuttoning the top button of his shirt in the car. When they cut to a reverse shot (with the camera on Sean's side of the car) his tie and shirt are done up again. The shot is reversed again as Sean is seen getting out of the car and proceeds loosens his tie while outside the car this time.
(at around 2 mins) When the boys are writing their names in the concrete, Jimmy starts his name at the top of the slab, by the time Sean has scrawled his name in, Jimmy's name has moved to below center of the slab.
(at around 2 mins) When Jimmy writes his name in the concrete, in the first shot the "J" is perfect and the "Y" is bigger than the rest of the letters. In the 2nd shot (when Sean is writing his name) the "J" is crooked and the "Y" is much smaller.
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 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.
Near the end of the movie, Jimmy has his hand on Sean's shoulder. In one shot Jimmy's jacket sleeve is at mid-wrist, exposing his sweater sleeve and watch. In the next shot, his jacket sleeve is pulled all the way to his glove, and his watch and sweater are not visible.