When Mullins is reading the teachers' quotes from Ashburn's yearbook she says that one of the teachers put, "Have a great summer," but later when Ashburn is reading her yearbook it clearly says, "See you next year."
(at around 58 mins) When Mullins takes Ashburn to her parents house. The family is arguing at the table and her brother enters the front door carrying a bag of shopping. The bag starts off with nothing in the top, then bread appears, then it disappears again.
Outside the interrogation room, Mullins complains about Ashburn's barrette so Ashburn removes the barrette and pulls her hair behind her right ear. Ashburn's hair repeatedly changes from behind her ear to covering her ear to partially covering her ear between shots.
When Mullins is busting the John in the Volvo, there are several shots of the front of the Volvo and the headlights are off. When Mullins pulls the john through the window and the headlights are visible again they are on.
When Mullins bursts into Jason's room at the end you can see behind the door there is nothing and no one. It flashes to Jason laying in the hospital bed then back to Mullins. As it does this it is clear the little door behind the room door does not appear to open, yet Larkin appears from behind it.
Levy tells Ashburn to go to the Boston Police's East Boston precinct, which is identified as G-5 in Area F; in reality it's A-7 in Area A. Also, a Boston Police district's identifier shares the same letter as the area it's in, so it should have been identified as either F-5 or located in Area G.
At the end of the movie when Ashburn and Mullins arrive at the hospital to save Mullins brother, Ashburn is left in the foyer as Mullins races to the ward the brother was in. Part way to the ward, Mullins finds out the brother has been moved to another ward. However Ashburn goes directly to the "new" room even though she wasn't present when the doctor informed Mullin.
When Larkin reveals himself, Mullins is openly moving her hands around and even tries to untie her partner, in plain view of Larkin. He somehow doesn't seem to notice this; and when Julian comes back Mullins goes back to pretending to be tied up.