During the love making/gun fighting scene, Smith is shown with his boxers still on and then suddenly off again.
At the scene Smith and DQ meet again, when DQ drops the ice cream glasses and start to kiss Smith she has her finger nails painted with pink polish, however on the next kissing closeup she hasn't no more.
In the shootout at Smith's house, the lighting changes dramatically from pitch black outside, to broad daylight.
When DQ is going down on the elevator, the shot comes back to her while she is still in it, but when you look at the bricks moving behind her, they indicate that she is going up as opposed to down.
After Mr. Smith's spectacular slide (on his back) through the used motor oil during the warehouse shootout, his clothes are pristine as soon as he is on his feet again.
Even if the bullets wedged between Smith's broken fingers had gone off that quickly when he put them next to the fire, which is highly unlikely, without a barrel to direct the energy they would have basically amounted to firecrackers.
When Smith and Hertz point firearms at each other, Smith tells Hertz his Desert Eagle pistol is a six shooter. However, a .357 mag DE holds 9 rounds, the .44 mag holds 8 rounds and the .50 AE hold seven rounds. This would not include an additional round that could be chambered in addition to a fully loaded magazine.
When Smith is busy shooting the bad guys in the warehouse, the last two shots he attempts to fire are met with the sound of dry-firing - a hammer falling on an empty breech. It is then shown to the audience that the slide is locked back, which occurs when the weapon runs out of bullets. In this condition the hammer cannot move, and the gun would not make any sound whatsoever if the trigger was pulled.
During the opening, Smith fires a Walther PPK at least 21 times before he reloads. The Walther PPK holds 10 rounds in 22lr, 7 in 32 ACP, and 6 in 380 ACP.
In the bathroom scene between Smith and the Lone Man, Smith has dropped his SemiAuto in the toilet bowl and Disassembles it on the changing table to clean and dry the weapon for some reason. The gun would have functioned fine if he had buried it in sand, swam under the water or frozen and then thawed it. After having performed the cleaning and drying, there was no reason that the weapon wouldn't function, and drying the Firing Pin / Transfer Bar area under the hand dryer made no real sense other than Hollywood cool. Yeah, yeah, I know it was just one of many goofs related to firearms in the movie, but as an instructor, this one just stood out. I enjoyed the romp for what it was though, just FUN...
Hertz drives around with the corpse of the baby's mother the next morning, at least several hours after she was shot to death, yet there are no signs of rigor mortis (stiffness) or livor mortis (skin paleness).
When the Lone Man reloads his S&W Model 629 revolver in the bathroom, the primers in the shells are dented, indicating they're dummy rounds with no gunpowder.
In the car scene where Smith shoots out the windshield of his car and the van full of assassins the glass breaks into a few large pieces which easily fall out of his way. Car (and van) windshields are made out of two pieces of glass with a sheet of polymer or laminate between. This means that when broken (even if shot) the windshield will break into many small pieces, but these pieces will be held in position by the laminate. The glass used in the two vehicles for this scene are obviously not 'real' windshields. Furthermore, in a head-on collision of two vehicles traveling at the depicted speed, Smith should have been propelled all the way through the van's cabin, probably hitting his head against the back door.
Throughout the movie, especially in long shots, the baby carried by Smith or DQ obviously has been replaced by a doll or dummy.
Despite the movie being set in New York City and its surrounding areas, the CN Tower (located in Toronto) can clearly be seen in an establishing shot of New York.
Hertz reminds Smith that his 9mm Pistol was empty while at the brothel: 8 on the rooftop (it was actually 11), 1 in the men's room, and the second cartridge was expended on the playground. As he mentioned the men's room (the scene prior), Hertz was nowhere near Smith nor in the same vicinity.
Mr. Smith and Donna bring baby food for the newborn during the days that he is hidden in the tank. A newborn baby does not eat solid food, ever, only breast milk or formula.
When Hertz claims that Mr. Smith's pistol is empty he states that Smith fired 8 shots on the rooftops. This is incorrect as Smith fires 11 shots during the rooftop shootout.
Smith incorrectly refers to Hertz's desert eagle as a "six-shooter." The term six-shooter refers to a revolver with a six round cylinder.