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.
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.
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.
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).
When John Lee is about shoot his mark (the son of Stan 'Zeedo' Zedkov), 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".
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.