When Nicholson guns down the bartender who had just been talking to Terry, the baseball bat the bartender is holding while being shot varies from being shot to pieces to being back in one piece again between shots.
When Terry and Kate meet in the bar with Jackie, Terry is smoking. The smoke can be seen when the shot is on Kate but there is no smoke when the shot is on Terry. This repeats throughout the scene as the camera cuts to each character.
In the final shoot-out, when Noonan jumps over the bar he is seen about to fall backside down, but when seen landing on the backside of the bar he lands face down.
The NY State vehicle registration and inspection stickers disappear and reappear from the driver's side of Jack's windshield; these would not be absent from any car legally registered in NY.
Near the end of the film when Terry goes to Jackies apartment to tell him that he heard Frankie was involved in Stevies death, the Tequila bottle is on the window ledge on its side. After the scuffle between terry and Jackie, the Tequila bottle is upright.
When Jackie shoots Cavello and his two associates in the bar he fires seven times with his favorite gun a Charter Arms Bulldog. The Charter Arms Bulldog only uses a five or six round cylinder.
During the final shootout in the bar, the slide locks back on Frankie's gun. After he's shot, he falls to the ground revealing the slide forward. Not such a big deal in itself but the slide locking back was really illuminated during the previous scene.
At the 2 hour point, Terry (Sean Penn) is seen walking down the street towards the bar for the final shootout. In these shots his hair is brown, long on top and curly. When he reaches the bar his hair is shorter, straight and dyed black.
When the drug deal goes bad in the opening scenes, Sean Penn shoots the two with a Smith & Wesson Model 6906. When they return to the car and he points the gun at his friend, the hammer is not cocked. The Smith & Wesson Model 6906 has a hidden hammer and as it is also a semi-automatic, the hammer would be cocked after firing.
When Terry and Jackie are shooting at beer bottles on the roof at the start of the film, Terry's bottle is noticeably delayed in shattering after he fires his pistol at it.
About half way through when Terry tells Kathleen that he is a cop, behind them an arm can be seen as it waves past for a couple of seconds.