When Mark Kaminsky goes to the Industrial Site to stage his death, the driver's side window is closed and once he uses the bolt cutters to open the gate to drive through, the window is now open. Once he has parked the car next to the fuel line the window is again closed.
After the fight in the alley (3 against Kaminski), the police car pulls up and screeches to a halt. After the conversation with the police officer, the police car is sitting at a different angle, with both headlights visible instead of the driver's side one blocked by the cardboard boxes in the alley.
In the fist fight in the womens dress store Mark punches someone who can clearly be seen wearing a plaster on his head, next time the camera is on him it has come off and then re-appears when he is on the floor.
At the 35 minute mark, Rocca introduces Joey to Max during a conversation in the office and the position of Rocca's left arm changes when he tells joey that Max takes care of things (all kinds of things).
Kaminski and Keller take a picture together at Patrovita's party. The picture that Keller later prints is different.
Bombing a police station would be an act of terrorism, and a federal crime. Handing the case back to the local police would not be an option for an FBI agent like Harry.
In the final shootout, an errant round strikes a revolving light fixture causing the party lights to start working. Unless the switch to the lights was flipped, the party lights should have stayed off.
When Mark passes the dollar note to the man in the elevator, the doors should have automatically started to open again instead of continuing to close.
At the shootout in the end, Kaminsky shoots one of the gangsters while lying down next to a bar counter. The squibs attached to the guy goes off before Kaminsky has aimed or fired the weapon at him.
In the shootout at the end, when the elevator descends and the mobsters shoot at the closed elevator door, the door then opens, but all the bullet holes in the back wall of the elevator have been made from outside to inside, not inside to outside as they would if the bullets had passed through the elevator.
When the team sent in to rob the police station of seized narcotics detonates a charge to break a safe, the sound can be heard at street level. Considering that the vault is below street level and ensconced within a concrete basement, it should not be audible at street level.
When Martin Lamanski's limousine crashes into the gasoline tanker truck (lorry), there is nobody in it.
The glass door of the clothes shop shatters before the guy actually hits it. A similar mistake was made in Remo Williams: The Adventure Begins (1985) & Death Wish 4: The Crackdown (1987).
The cassette Kaminsky inserts into the tape player before the Gravel Pit gun battle is The Crusaders' "Street Life"; the music played is The Rolling Stones "Satisfaction".
(At about 35:20) When the female jogger walks up to the car window, she sticks a revolver in the thug's face. We hear the hammer cocked, but her thumb is nowhere near the pistol's hammer. Then in the very next shot, the hammer isn't cocked either.
When Kaminski and Monique are walking to Patrovita's house for the party, a bank of film lights and the boom operator are reflected on the doors and windows of the limousines they pass by.
Towards the end of the movie, Petrovita's car is "heading south on Route 84". Earlier, Kaminski tells Monique to go to the Rock Falls Airport and charter a plane. Route 84 in Illinois runs from Wisconsin to the Quad-Cities. Even assuming he drove all the way to Wisconsin to take that route, it never goes to Rock Falls.
One of the three men who jumped Joey was the shooter in the backseat of the car when Joey & Max Keller went to whack Martin Lamanski (then later backed off because there were too many people), yet he doesn't recognize him from the alley.
There is actually no reason for Kaminsky (Schwarzenegger) to fake his own death using the refinery explosion. The mob is not aware of him and do not know him at all. He could have gone undercover without all that.
During the raid on the informant's hideout, the last FBI agent killed shoots his gun as if has never handled a firearm before, just shooting erratically without even aiming.
At 49:15, a character is playing golf and a man is talking while the other character is playing. In the rule of golf, you never talk when someone is going for a swing because it can break their concentration.