The Silver Shamrock commercial halfway through the film says that it's two more days to Halloween. But the next night as Challis and Ellie are lying in bed, he hears the commercial say there are three more days to Halloween.
(at around 8 mins) When Harry Grimbridge startles the gas station attendant by grabbing him from behind Harry's hair, face and neck are soaked by the continuing rainstorm. Somehow his hands, clothes, and the Silver Shamrock mask he is clutching are completely dry.
Towards the end of the movie, Challis's shirt goes from untucked to tucked several times between shots.
When Dan is peering through the ventilation grate at the control room, we see Cochran hanging up the phone. In the next shot (close-up) Cochran still hasn't put the phone down.
(at around 1h 19 mins) When Dr. Challis throws the skeleton mask up onto the security camera, it turns around between shots.
(at around 50 mins) When the process of making masks is explained. It is said that the latex is heated and poured. If they are using liquid latex to make the masks then no heating is necessary. If they are making foamed latex (although the masks just seemed to be regular latex) then baking would be used once the foamed latex is poured but not before.
(at around 1h 11 mins) There are obvious timezone errors when they label the different cities. Ohio and New York (Eastern) still have daylight, while Washington and Arizona (Pacific/Mountain) are already into the night.
When Challis and Ellie spend the night in the Rose of Shannon motel room, there are three areas- the bathroom, a seating/dining area next to it, and the bedroom. As he is pursued later in the movie by the suited men he frantically climbs out of the bathroom window to the backyard. To the left of the bathroom window is a door, which means Challis could've ran through the seated area and out the door itself.
When Marge Guttman sets off the Silver Shamrock badge, the resulting laser is "tethered" to her mouth as she pulls back, instead of shooting in a straight line where her hand is pointing it.
(at around 16 mins) Dr. Challis and some bystanders draw closer to the spot where the suited stranger and his car went up in flames. Talking among themselves you can hear one of them say, "Somebody call an ambulance," but this all takes place in front of a hospital.
In the morning shot of the assassin's burnt-out car outside the hospital, it is obvious the parking spaces are at a regular ninety-degree angle, however the car is parked at a forty-five degree angle across several spaces. While this is plausible for the robotic assassin, the two cars beside it - obviously owned and parked by humans - are also on a forty-five degree angle instead of properly in the spaces.
(at around 1h 12 mins) Teddy, the coroner's assistant, finishes writing down the number for the Rose of Shannon motel from the phone operator before he provides her with all the digits.
(at around 23 mins) When Ellie takes Dr. Challis to her father's shop and discusses his record keeping the days before his death, the mask that he was clutching that fateful night is displayed in his store as "Silver Shamrock Novelties Glow-in-the-Dark Halloween Masks." They are also advertised similarly in the commercial airing at the gas station in the beginning of the film. On Halloween the children from all over the country are shown trick-or-treating and although dusk has already fallen on the children in Los Angeles, Seattle, and Phoenix, the masks do not glow whatsoever.
(at around 15 mins) Shortly after the suited man enters his car, douses himself with gasoline and strikes a lighter, you can see a dummy in the driver's seat before the vehicle explodes.
(at around 1h 17 mins) The scenes in the movie Halloween (1978) on TV around 7:45 p.m. are too late in that movie for it to be followed immediately by the big giveaway at 9. Even allowing time for commercial breaks, the broadcast should still be in the first half of the movie.
As Challis is talking to his ex-wife on the telephone (just after the hospital assassin has blown himself up), there is a fire officer in the room nodding his head at various points. The red and white "F" on his helmet can be seen bobbing around as if it isn't fixed to the helmet at all.
(at around 17 mins) After Dan hangs up the phone with his ex-wife in the start of the movie the police officer uses the phone and if you listen carefully you can hear "if you need help hang up and dial your operator," and it still happens as the officer dials.
When Dan is sneaking into factory after Ellie has been taken, the shadow of the camera and the camera man's arm are fully shown in shadow on the brick wall.
(at around 1h 11 mins) When the children are seen from different cities buying or wearing the Silver Shamrock Halloween masks the child on the bicycle in Omaha, Nebraska passes by a business called "Landis Dept. Store." It was a general store actually located in Los Angeles, California that closed its doors eight years after the movie was released.
The technicians at the Shamrock factory are still making the deadly computer chips and masks and working on the Blue stone an hour before the Big Giveaway at 9. These chips and the thousands of others in boxes around the factory will never be used as Cohran's goal will have been achieved after 9 pm.
(at around 33 mins) In what appears to be at least an hour or so after curfew is put in effect, Challis goes to a liquor store. Even if he didn't know about the curfew, it is not explained why the liquor store would still be open.
(at around 1h 15 mins) Just like on October 29th when Challis and Ellie initially booked a room at the Rose of Shannon Motel in Santa Mira, on 7:30PM Halloween night the liquor store in town is still open past the 6PM curfew.
The film begins with the chronological date of October 23, 1982. When Ellie Grimbridge tells Dr. Challis about her late father's whereabouts before his murder she states his last appointment was a dinner date that he never attended on October 21st, the day after his visit to the Silver Shamrock factory to "pick up more masks." This means that Conal Cochran had three days to terminate her father after he discovered the company's fatal schemes- but he only detained him and he escaped.
How did the robot assassin know where to find the drill that he used to kill Teddy? He is shown finding it in the first drawer he opens.
(at around 47 mins) The factory worker tells Ellie that he loaded her father's order into a green station wagon, but when we see the car later, it's blue.