When Danny (Donnie Wahlberg) is leaving Southie towards the end, he boards a #7 bus at Broadway station, however seconds later, he's seen traveling in a #11 Bus
At the beginning when the family goes to get Cathy, as they are leaving, Cathy and her friend are seen running down the steps with the bowling shoes still on. Once they are outside, you can see they both have on tennis shoes.
When Danny is leaving the alley after the gunfight he has a limp. Remarkably, he recovers and walks away in a brisk walk.
The news paper the crew read inside the bar with Colie Powers story has the day Friday March 15th 1997. March 15th 1997 was a Saturday not a Friday.
When Danny arrives at the Whitey crime scene, the police let him under the police tape, into the crime scene and allow him to physically touch the body. In reality this would never ever be allowed since it would compromise the crime scene in every way imaginable.
During the singing of "Southie is My Hometown" the lyric "raised up on D Street" in the second verse is incorrect. The correct lyric is "raised up on B Street."