During the carwash shootout, the car that John and Meg jump into becomes riddled with bullet holes. Whenever the camera angle switches to the more distant view, as they drive off, the bullet holes are gone.
In the opening scene, John Lee shoots a gangster in the chest, leaving bloody entry holes. As the gangster falls, however, his yellow jacket opens to reveal an unstained green shirt with no bullet holes whatsoever.
When John is sitting in the photo booth he closes his eyes for the final picture. Later, when Meg is fixing the passport, we see the three unused photos as well as the one that she uses, and in none of them are John's eyes closed.
In the opening scene, as John draws the second gun and twists around to shoot the last bodyguard, his tie ends up over his right arm as he shoots. In the overhead view after his last shot, his tie is back as it should be, but he hasn't moved.
The MP5k and the Beretta PM-12S that the 'Replacements' use fire continuously for long periods (or in bursts many, many times). An MP5 will empty its 30-round magazine in 2.3 seconds on full-automatic fire.
After Meg finishes doctoring Alan's passport for John, he opens it up to check her handiwork. When he closes the passport, the name on the front is "Alan Chin," however, the character's name is listed as "Alan Chan" in the credits.
After John and Meg escape from Wei's gang, they return to John's hideout, where he treats her dislocated patella - which looks amazingly like the real thing, makeup or no. John pops the kneecap back into place with no difficulty. However, anyone who has ever suffered that particular injury knows that it would be impossible to immediately stand, walk, run and jump as Meg does when the evil-doers subsequently chase them from the apartment. The tissue damage involved takes weeks, if not months, to heal enough to allow that kind of activity.
During the shootout at the arcade, Ryker uses an MP5K in a briefcase. If the briefcase were actually used in this manner, the shooter would most likely end up with a calf-full of lead. The proper technique of employing an MP5K in a briefcase (as taught by the HK International Training Div.) is to hold it with both hands and brace it against the chest/stomach (muzzle toward the target, of course).
At the airport when the different incoming and outgoing flights are shown, flight RK 27 from Melbourne and Flight NK 611 to Sydney are incorrect. Flight RK was owned by Royal Khmer airlines (started operations in 2000, 2 years after this film was made and never flew in Australia.) Flight NK is owned by Spirit Airlines and likewise has never flown in Australian airspace.
When John Lee is about shoot his mark, the scope cover on the front of the scope is not the correct type of cover. Typically, the red buttoned scope cover is used at the rear side of the scope. Butler Creek makes these type of scope covers and the red buttoned one's are designated as "rear scope cover".
Alan's passport is shown to be a British Citizen (GBR) passport. Given the time the film was made and the time the older blue British passports were retired, Alan would have had a Hong Kong (British National Overseas) passport and not a British Citizen one.
In the final scenes after the shoot out at the end, when Zedkov runs into John and Meg in the stairwell, Chinese characters are clearly visible all over the walls. However, in a few cuts during the confrontation, a two syllable Korean word is clearly painted on the walls. The word is "bah-boh" which loosely translates to "Dummy" in Korean.