Factual error: In the early part of the movie, there is a shot of the outside of Triple O's bar. The scene takes place in 1975 and "Coors" is advertised on the outside of the bar. Coors beer was not available in Boston/East Coast in 1975. It was only available from the 1980's.
Continuity mistake: When Jimmy agrees to play gin with his mother, she goes and opens a drawer and grabs a blank pad for scorekeeping with her left hand and a deck of cards with her right hand. In the next shot, she emerges from the drawer with her right hand empty and tells Jimmy to "get the deck."
Continuity mistake: When they're walking with Tommy, Jimmy is talking with him before Martorano kills him. They stop walking and Tommy puts his left hand on his hip. In the next shot his left arm and hand are straight down at his side.
Continuity mistake: When Tommy opens the car door to confront Officer Flynn, the officer stands straight up. As Tommy is pulled back into the car and closes the door, Officer Flynn goes back to leaning over and rests his hands above his knees on his inner thighs. When the shot changes his hands are on his outer thighs.