Corrected entry: When Roger and Peter return to the truck yard to hotwire the second truck, if you look outside the front/driver's windows of Peter's truck, you can see a van driving quite close to the action. The four main characters are the only "living" people for miles, thus the van should not be driving around. (Unless some zombie suddenly got his driver's license.) (01:01:30)
Corrected entry: When Stephen is being attacked by the zombies in the elevator, Peter is able to hear it over the radio. As shown earlier in the film the red button must be pressed down to communicate.
Correction: Peter doesn't hear the attack over the radio, he hears it carried through the air ducts. Remember that the top of the elevator was open and near an access to them. Peter was going to help Stephen out of the elevator when his screams carried through shafts. At this point we don't know how exactly how close Peter is to the elevator and it's possible he was close enough to the attack to hear it though the ducts.
Corrected entry: If Fran was 31/2 or 4-months pregnant like Steve said, then she shouldn't have been able to lift them heavy cardboard boxes to block the door.
Correction: Being pregnant does not make you weak; not lifting heavy objects is only a recommendation, not a physical law. Many women in poorer countries work heavy manual jobs whilst pregnant, often up to the day they give birth. So there is not reason why a fit, healthy, 4 month pregnant woman could not move some boxes, especially in such extreme circumstances.
Corrected entry: In the sporting goods store, Peter tells Stephen not to try & shoot the zombies through the grill at the storefront - ".the openings are too small. bullets will wind up chasing us around in here." However, the camera cuts to the grill, which has openings of about 1.5" - well large enough to shoot through. And since the grill is made of a light metal, bullets could not possibly bounce/deflect off it. (01:10:30)
Corrected entry: How did the janitor zombie that attacked Roger in the JCPenney know to stand still with the mannequins before he attacked Roger? I thought the zombies weren't supposed to be smart. Also if the JCPenney was secured, how did the zombie even get in?
Correction: Zombies have some limited intelligence, such as when the zombified character remembers where the passageway has been covered up. Also look at the original Night of the Living Dead; various zombies were intelligent enough to use weapons. As for how the zombie got in, he could have been maintenance, a cleaning crew, or management who was working late.
Corrected entry: When Peter aims at the biker with the machine gun, by the time he actually shoots him, barring a shot that defies the very laws of physics, he manages a physically impossible shot, given the position of Peter(on the second floor), and the biker (outside). (01:55:10)
Correction: The physically impossible part, I assume, comes from the angle. This is not true, however. We do not know the relative location of the duct from which Peter shoots. It is very possible that he is far enough away to shoot at an angle shallow enough to hit the biker.
Corrected entry: When the screwdriver zombie grabs Roger and Roger sticks the screwdriver into the zombie's ear, you can tell that the screwdriver is retractable.
Correction: I have watched this scene many times and I don't think it is obvious it's retractable. I'm sure that in the movie-making world that is how it was done but it's not visible.
Corrected entry: When the SWAT team is raiding the building in the beginning, one of them knocks down a door through a barricade. Immediately many zombie arms shoot through and force the barricade off the wall. If the zombies were close enough to reach through the barricade, they should have been knocked back by the falling door, and they had no time to approach. (00:14:00)
Correction: The Swat Team does not break the door down, rather, a shotgun is used to break it open. You can see it swing open after being struck. Once it opens, there is a cut to the guy who opened the door leaning down to look through the boards. Then hands start reaching through. The zombies had plenty of time and space, considering.
Corrected entry: When Peter and Roger overlook the mall from the 2nd floor 2 zombies can be seen in the top middle of the screen on the 2nd floor, although Roger said there weren't any zombies on the 2nd floor. (00:29:40)
Correction: There are two things wrong here. Firstly, Roger said, "I haven't seen any of them up on the second floor." So if there were zombies, he just hadn't seen them - a character (not movie) mistake. Further, in the four shots of the mall proper from the roof, there are no zombies on the upper levels.
Corrected entry: After Steve, Roger and Peter have secured the Mall so that no more Zombies can enter they than decide to kill the few remaining zombies left in the building. One of which is a zombie that took Rogers M-16 assault rifle. However at the end of the movie after the Raiders open the doors we see the same Zombie walking around and even gets to be in the final rooftop scene. He should have been killed and disposed of earlier because he was locked in the building, not out. (00:38:05 - 02:05:40)
Correction: During the scene where the semis are being put in place, Roger forgets his tool bag. When they return to retreive it, the zombie with the M-16 can be seen. This means he is outside. The doors were too crowded to allow him entry before they were locked, very soon after this scene. Therefore, he remained outside until the raiders came.
Corrected entry: In the store room at the top of the shopping mall in which all of the characters live, sleep etc. if you look at the boxes you will notice that the best before date on some of them is May 1962. The film was set in 1978. I doubt very much if a shopping mall would stock items which are 16 years out of date.
Correction: According to the co-writer of the book "The Living Dead," which Romero was working on at the time of his death, there is a 5-year span between "NOTLD" and "DayOTD," with "DawnOTD" occurring 3 weeks after the initial infection of "NOTLD."
Correction: Keep in mind that Dawn of the Dead is the sequel to Night of the Living Dead. In Night of the Living Dead, the film is set in the sixties. The events of Night of the Living Dead takes place in 1968; no exact date is given as to when the zombie invasion occurred.
Correction: There's nothing in the film that explicitly states that the survivors in the mall are the only humans around. The van could very well have other survivors trying to get away from the zombies.
Very plausible and realistic explanation, but the fact is that this is a blooper/mistake just as OP suggested.