(at around 23 mins) Lindsey's socks when she leaves the room after getting the sugar are white. When she comes back to the room, her socks change from white to red.
(at around 1h 9 mins) When Slevin walks through a hallway to The Fairy's room, there is a part of his hair on top of his head that changes from being flat on his head to standing up various times.
(at around 37 mins) When Slevin is eating the first sandwich he eats in the movie, it changes from being half eaten to having only one bite taken out of it.
(at around 1h 1 min) When Slevin and Lindsay are walking back from their date and talking about picking up a pigtail, there is an extra behind them that disappears and reappears several times.
(at around 37 mins) Also with the sandwich, the eaten bites rotate from being the bottom corner to the top corner on multiple occasions.
(at around 47 mins) During one scene, Slevin tells Lindsey that he has "ataraxia," a condition he describes as "freedom from worry." But the English subtitles have the word showing as "anorexia," an eating disorder.
(at around 1h 12 mins) 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.
(at around 9 mins) 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.
(at around 1h 1 mins) When discussing Bond villains, Slevin refers to Ernst Stavros Blofeld. The character's middle name is in fact Stavro, without the second S.
(at around 56 mins) When Lindsey tells Slevin, she is a coroner. Slevin calls Lindsey a "weirdo," but the English subtitles have the spelling incorrectly showing as "wierdo."
(at around 1 min) At the beginning of the movie, when Slim Hopkins gets shot in front of the door of his car, the window's glass is already broken before the bullet goes through him into it.
(at around 1h 3 mins) Josh Hartnett screws up his line when he says "Because if you don't if you aren't talking about the guy..."
(at around 11 mins) 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.
(at around 4 mins) During the opening credits when Nick is walking towards the camera to sit down, you can see the reflection of the camera man moving around in the back of the front left blue chair.
(at around 22 mins) While Sleven and Lindsey are talking about what happened to Nick, Lindsey picks up the apartment phone and presses the spring button twice and connects to the last caller. That action would only result in a dial tone.
(at around 9 mins) 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.
(at around 9 mins) 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.
(at around 47 mins) Slevin tells Lindsey that he has 'Ataraxia' which is a condition characterized by freedom from worry or preoccupation. The entire movie is based on his preoccupation with revenge.