After Jimmy's mom dies, and he and his brother are at the kitchen table, his brother's glass of beer is about half full in the first scene, and about 3/4 full in the next scene.
In the film, Buddy Leonard diffuses a fight between Whitey Bulger and Tommy King at Triple O's. Tommy is subsequently murdered, and Buddy is found dead in the front seat of a car a few scenes later. In real life, Buddy was murdered before the altercation at Triple O's. His death was the real reason Tommy attacked Bulger.
In the movie, Whitey Bulger recruits his partner, Steve Flemmi, into becoming an FBI informant with him. In real life, FBI agent Paul Rico recruited Flemmi 10 years earlier.
When Jimmy and Stevie are waiting for Deborah to come out of the police station, you can see the reflection of Juno Temple (waiting for her cue to come running up to the car) in the window behind Jimmy's shoulder. This happens just as Jimmy says "Don't you f***ing correct me" and before Deborah is seen coming out of the station in the wide shot.
Near the end of the movie: Billy is on the phone (yellow phone) with Jimmy, Jimmy is using a payphone in a phone booth. Billy's phone is missing the cord from the receiver to the base of the phone.
In 1975, the outside of Triple O's bar advertises Coors Light beer. Coors Light was created in 1978, and was available on the East Coast in the mid-1980s.
During a restaurant scene in Florida where a duffel filled with $20,000 is handled, a bottle of Jalapeño Tabasco Sauce is on the table. Tabasco released that flavor in 1993.
Many shots include the yellow glare of sodium streetlights, which were not in widespread use until the late 1980s.
Al's Liquor in South Boston features a mural of Dropkick Murphys, a band that formed in 1996. The mural was created in summer 2000.
The Angiulos lived at 98 Prince Street in Boston's North End. The street shown is a nearby side street.
John and Billy both say that if they could have any baseball player it would be Gary Carter, catcher for Montreal. In 1975, Boston's catcher was Carlton Fisk, who would retire with more home runs than any catcher in history. If the Sox needed to improve any position, it wasn't catcher. Fisk made the Hall of Fame on his second ballot. Carter got in on his sixth ballot.