Towards the beginning of the film at the horse track they show a horse fall during a race in 1979. In one of the scenes you can see a new John Deere tractor in the upper left hand corner of the screen.
When Max and his son are in the car discussing their
cover story, they mention that Tony Taylor was the second baseman for the Phillies. In 1979, Taylor did not play for the Phillies any longer. Manny Trillo was the everyday second baseman.
When Max and his son are in the car discussing their cover story, Henry mentions that one of the players was Jesse Gonder. This flashback takes place in 1979, but Gonder played his last MLB game in 1967.
When Slevin is first taken to see "The Boss", as he is exiting the building, someone is taking a picture of him. You see his reflection in the lens of the camera. The reflection should have been reversed, but it was not.
When Lindsay leaves Slevin (after their initial meeting) carrying a glass cup containing sugar, the cup is only 1/4 filled. When she (immediately) returns, to see if he has, again, removed his towel, the cup is about 2/3 filled.
In the beginning of the movie, after Slevin has finished shaving in the bathroom, he uses a towel to wipe off the excess shaving cream. He still has some shaving cream on his neck when someone suddenly knocks on the door. We immediately cut to a side shot and Slevin turns his head toward the knock. He no longer has any shaving cream anywhere on his neck.
Goodkat (Bruce Willis) when sitting in the wheelchair at the beginning of the movie has no hair under his hat from the top of his ears downwards. When he appears later in the movie he has a full head of hair.
When Slevin and Lindsay are in bed discussing the different James Bonds, Slevin's arm is underneath Lindsay, her arm is across his chest and they are facing each other. After she says, "Well, on that we agree," suddenly Nick is laying flat and looking up, her arm is not on him, and he is touching his necklace. Then the camera angle changes back, they are facing each other again, but her arm is not across his chest. Just after she says "Scotland forever," the camera angle changes again, and suddenly Slevin's arm is not under her.
During the scene in the beginning where the guy gets shot in the parking garage; as he approaches the car you can see the driver side window is already shattered. After he gets shot he puts his head crashing through the already shattered window.
When Goodkat is shooting two silenced pistols at the men coming through the wall, the pistol in his left hand has a bent silencer. It is obviously bent down at about 15 degrees and would be dangerous to the shooter if shooting anything but blanks.
When Max and his son are sitting in the car at the racetrack, the vehicles are parked too close together. There is only about eight or ten feet between rows of cars. There is no way a driver would be able to maneuver a car out of a parking spot.