At the scene when Wesley is told to shoot the wings off the flies, Gunsmith holds a gun to his head. When he cocks the hammer back, there is a sound clip of a gun having the slide manipulated, not a click of a hammer.
Wesley's first assignment, Robert Deane Darden, is described as a 48-year-old man who weighs 180 kg. (396 lbs.). It's strange that his height is given in imperial, and his weight in metric. Also, a 6'1" man weighing 180kg would be quite massive, either very muscular or very obese, whereas the man Wesley's shown to kill looks quite average. Considering other targets' weights are given in pounds, it's more likely this should have been recorded as 180 lbs - which would be quite reasonable.
When Wesley is in his father's apartment having everything explained to him, his hair is wet from the bath. In one shot by the window hair is sticking up on either side of his head, the next shot it is fixed, and the next shot it is sticking up again.
When Wesley is in the Mustang going after his target, he fires 2 shots into the bulletproof glass. When the shot cuts back to him when he says "Shit" , the slide on his gun is clearly locked back, signifying he is empty. His gun of choice is a Heckler & Koch P2000 9mm Compact. This model has a 14 round capacity. Even a novice assassin like him would ensure is gun was fully loaded when "hunting" as a contingency.
When Wesley motions for Fox to speed up in the Corvette she "up-shifts" then the camera shows her foot on the accelerator pedal, showing a brake pedal for an automatic transmission equipped car, not a manual.
When Mr. X is first being attacked there are three shots fired one at the woman and two at him which shatter the glass doors. Although if you look closely when he jumps through the window there is only one bullet hole in the glass.
In the scene where Fox is driving the Dodge Viper, she rams the police car into the barrels and gets water sprayed all over the her windshield. In the next scene the windshield is spotless and there are no streaks to indicate she hit the wipers.
During the chase sequence with the Mustang and the C4 Corvette, a camera shot briefly shows Fox's hand down-shifting from above. It shows the shift lever from the Dodge Viper used earlier in the movie.
In the scene after Wesley quits his job, Fox is outside waiting in an Audi SUV. When driving, there's a close-up of the instrument panel, focusing on the knife used to steal the car. Although the car is supposedly driving, the speedometer is at zero. Also in that scene, you can see the knife's blade is actually bigger than the ignition lock slot's width.
(at around 59 mins) Fox (Angelina Jolie) character is seen emerging from the water, just as the male character had (whose hair was soaking wet). A few seconds later, her figure is revealed in focus, hair clearly completely dry, and bunned.
When Wesley meets Sloan and company, Sloan tells him they've transferred all his assets to Wesley. When Wesley goes to his ATM to check his balance, he withdraws a large sum of money. However, when he is later viewing his account online at work, his balance displayed is exactly the same as before he withdrew the cash.
When Wesley is looking at his new account balance on his computer at work, Barry comes up behind him and whispers, "Wesley." Wesley spins around in his chair and grins at Barry, but in the next frame Wesley is still facing his computer and is looking over his shoulder at Barry.
Wesley hits Barry in the mouth with a keyboard, and a lower molar flies out of Barry's mouth. When Wesley goes back to get his father's gun, his upper incisor is missing. While it could be argued that Barry lost more than one tooth, but the teeth are in different regions of the mouth, and Wesley hit him once.
Just before Wesley and Fox jump over the roof of the train for the first time, Fox is wearing high-heeled boots. After they jump, Fox is wearing flat boots. Also, just before Fox ducks under the bridge, she's wearing high heels again. This was possibly intentional due to the "sexy look" of the high heels, but short heels were obviously more practical.
After Wesley quits his job, Fox is picking him up in an Audi Q7. When driving, you can see both of them through the passenger door window. Although the car is moving, the reflection in the mirror cover doesn't move - while every other reflection (e.g. in the window) is moving.
The code in the assignments change. It seems to be a trivial ASCII/binary code (00001 = A, 00010 = B,...) but the code for E changes from 00101 to 00011 in the first assignment, when Delane is being chosen as a victim.
The movie is suppose to take place in Chicago, but when Wesley drives the Mustang and Fox drives the C4 Corvette to assassinate the man in the limousine, they race through Long Island City, Queens. The Citicorp building and the New York City skyline are in the background.
While undeniably cool looking, the multi-component bullets used in the sniper rifles and Cross' gun would actually have less range than a standard full metal jacket. As the bullets disassembled themselves in flight, each part that fell away would reduce stored kinetic energy imparted at the time of launch. High kinetic energy is a vital component to range, which is why a heavy 4-inch artillery shell flies further than a .50 cal bullet, which flies further than a .22.
When Wesley types in his name in the search engine, www.google.com, he enters it without quotations, and gets no results. However, in order to get no results for a name, you would have to put the name in quotations, like "Wesley Gibson" - assuming there is someone by that name. If you only enter the name without quotations, like in the film, the search query will yield any results for Wesley, and any results for Gibson, (Wesley + Gibson as opposed to "Wesley Gibson") making it factually impossible for a search engine to yield zero results.
At the train station, a door leads to an elevator with a sign that says "An elevator with an operator" in Czech. The accented letters in "operator" are wrong. It was most likely translated on a computer without Central European fonts, a common mistake.
Every time we see a bullet fired from modern guns fly through the air, it has 3 shallow grooves in it. Most modern firearms have 5-6 sets of lands and grooves, depending on the manufacturer. The Heckler & Koch that Wesley uses for most of the film has 6 sets of hexagonal lands and grooves, whereas the Beretta that Sloan uses to show Wesley how to "curve the bullet" has 5 sets. The Olympic Arms Matchmaster that Fox uses also has 6 sets, and would definitely "score" the bullet accordingly, therefore making the "Goodbye" on her last bullet impossible to read.
It's a little strange that the fraternity uses binary code, especially since the binary code that they used correlates to the ASCII chart (e.g. '01101000' translates to an 'H'). The first ASCII chart was created in 1963, and the version they use was updated in 1986. Binary code is perfectly reasonable given the explanation of how the cloth is "read". The use of ASCII specifically is strange, but any speculation on this point is moot given events that transpire later in the film.
The Fraternity would not have called themselves that 1,000 years ago. The word first appeared around 1309, as the Middle English word "fraternite." In fact, the founders would have spoken Old English, which has more in common with Old Norse than Modern English. The closest equivalent would probably be a word based on "broþor", the Old English for "brother" (as in brotherhood). The founders would not have spoken Old English. The "birthplace" of the Fraternity is revealed to be Moravia, part of the Czech Republic (where Wesley goes to confront Pekwarsky). "The Fraternity" would simply be a relatively modern English translation of the original Czech name.
As Fox is beating up Wesley, there is a punch that is divided into three shots. In the first shot, she starts the punch with her left fist, which is bare. The next shot shows her other fist, the one with the brass knuckles, coming at the camera. The third shot finishes the punch, with the bare left fist. It is more likely that the sequence was a deliberate attempt to show the speed of three separate punches.
After quitting his job, Wesley sees his and Fox's photo from the security camera, at the pharmacy, on the all newspapers' front pages. Once the incident at the pharmacy occurred, the police would have questioned the pharmacist who had Wesley's name for the prescription (and most likely his address also). The police would have picked him up before he went to work for questioning.
When Sloan is showing Wesley the loom, he puts a clipboard on the fabric, which clearly demonstrates the fact that while the machinery's working, it's not really doing anything since the fabric isn't moving.
When Fox picks Wesley up after he quits his job, there is a close-up of the knife used to steal the car. The knife-blade is translucent and the entire ignition slot through it. Additionally, despite the fact that the car is moving, the speedometer is set at zero.
The goof items below may give away important plot points.
In the last scene where Wesley sets up Sloan, when the camera shows Sloan looking down, you can see a trashcan to the right. However only seconds before when the camera is approaching Wesley's desk, you can see there is no trashcan at all.
In the last scene, when Wesley sets up Sloan, when the camera shows Sloan looking down, you can see the cross made out with post-it notes, however only seconds before when the camera is approaching Wesley's desk, you can see the floor but no post-it notes.
When Wesley gets his first assignment from Sloan at the Loom of Fate, he reads a sheet of paper that Sloan has just deciphered. A few seconds later, Wesley looks at the paper and it shows the target's description. If Sloan had just deciphered this name, how is the description part of the paper already filled in? This is either a continuity error, or the first evidence that Sloan is manufacturing the names of the targets. Also, Sloan writes most of the binary code quickly without checking the cloth. This further implies that he knew the details of the "fated" target in advance.