Factual error: In a scene where the nightclub singer with the tropical fruit headwear is on stage, she is singing into a classic RCA 77D microphone which is correct for the period, but in a mistake often made in movies that are set in this era, the microphone is wrongly positioned. It is leaning toward the singer when in fact this type of mic is spoken to from the side, not the end. To work correctly it was always pointed away from the singer or placed in a vertical position for side pickup. (01:06:15)
Continuity mistake: When the squad arrives at the hotel to arrest Mickey, they are on the street with two cars behind them, but in the upper view those cars disappear. (01:33:25)
Continuity mistake: When Cohen walks in the ruins after the Gangster Squad torched his call center, Eugene removes his hat and is looking at the damage. The shot changes and he is looking at a slightly more downward angle than before. (01:08:45)
Continuity mistake: In the scene where Sgt O'Mara rescues the girl from Cohen's thugs, when the Murphy bed closes, she is head-down. When he opens the bed, she is standing right-side up.
Chosen answer: No, this is explained when O'Mara recruits Kennard. Kennard says that nobody would work with Ramirez because of his Latino heritage, so Kennard took him under his wing. And while he did that, he also taught Ramirez to shoot just as good he can, because before that, Ramirez was a lousy shot.