(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.
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 28 mins) Ellie and Rafferty stand outside the hotel while Dan goes to check in. The sunlight is seen brightly shining on the windows of the hotel as Rafferty draws attention to Mr. Cochran's car. The scene cuts to an interior shot of Rafferty and Ellie as seen from Mr.Cochran's point of view inside the car. Looking at the windows of the hotel it's obvious the sun is at a different position from where it was just a moment before and the light of day has faded.
(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.
(at around 1h 10 mins) During the "test" on "Little" Buddy, after the family has all been killed the camera switches angles from an in room shot to a monitor shot. In these shots it's clear "Little" Buddy's legs have changed their position several times.
(at around 1h 8 mins) When Little Buddy and his family are being murdered by the use of Little Buddy's Pumpkin mask, the Silver Shamrock tag on the back of the mask disappears and reappears several times.
(at around 37 mins) As Ellie Grimbridge meets and talks to Marge Guttman outside the motel: Ellie's jacket is open. In the following shot when Marge shows Ellie the displaced Silver Shamrock trademark, her jacket is nearly closed. Yet, in the next shot, Ellie's jacket is once again open.
(at around 28 mins) When Dr. Challis and Ellie initially check into the Rose of Shannon motel, the signs displaying the room numbers are a deep red. (at around 54 mins) After taking a tour of the Silver Shamrock factory the next morning and deciding that they should depart to find some help, Challis goes to the motel office to use the telephone. On the way out he leaves their door ajar and the room number, 6, is now prominently displayed on a faded pink sign. On the way to the office and returning to the room, all the signs are deep red again.
(at around 1h 15 mins) As panoramic views of the liquor store, the Rose of Shannon motel and the Silver Shamrock factory are shown, the current time in the movie is shown as 7:30PM. When the next scene immediately cuts to Cochran looking over a chair-bound Dr. Challis, the time on the digital clock above the door says 7:46.
(at around 30 mins) After Dr. Challis checks into the motel, almost gets hit by Little Buddy Kupfer's bicycle and narrowly ran over by Marge Guttman's car, he enters the room and tells Ellie the "place is a zoo". Although it was bright and sunny outside immediately before, when he enters it's pitch dark behind him.
(at around 9 mins) Dr. Challis visits his children at his ex-wife's house with surprises for them in brown paper bags. His children take the bags, step aside, and you can see his daughter swiftly push her hand in and pull a mask out. In the next shot, the bags are still rolled up and she quickly puts her hand in and pulls the mask out again.
(at around 27 mins) When Dr. Challis and Ellie arrive in Santa Mira, they drive by the glaring townspeople and make a u-turn near the church. Parked on the left side of the street are three cars. When Dr. Challis and Ellie stop on the next corner by the factory, there is only one car parked in the distance.
(at around 25 mins) Dr. Challis and Ellie begin their travels to Santa Mira. As Challis reads aloud about the small town, the car goes down a road and makes a right turn. There is no one in the passenger seat beside Ellie. When they approach the town sign after, Challis is now visible in the previously empty seat.
After the physical altercation with Cochran's robots and his diversions in nearby bushes to rescue Ellie after she was accosted, Dr. Challis' jacket becomes soiled- especially the back as seen in the elevator when Cochran takes him to the final processing area. After he escapes the holding cell, his jacket is clean and spotless up until he finds the gas station- then his jacket is dirty again.
(at around 34 mins) After being startled by Starker and complying to his request to have a drink from his bottle of alcohol Challis wipes the opening using his handkerchief with one hand, takes a swig himself with the other hand, and winces from the liquor. As Starker talks about Conal Cochran you can suddenly see the silhouette of a cigarette in Challis' mouth then shortly thereafter hear the flick of a lighter and see him light the cigarette.
(at around 29 mins) In the split second that Little Buddy's bicycle falls from the Kupfers' RV and Challis picks it up, the motel office sign goes from the letters being fully obscured by the sunset's shadow to the sign being fully illuminated by the sunlight.
(at around 1h 16 mins) When Conal Cochran begins to explain his motives concerning the Halloween masks, the wisps of hair on Challis' forehead have changed position although he is immobile from being tied up.
(at around 43 mins) When Challis and Ellie are awakened in the middle of the night by the sound of the vehicles pulling up to the motel, they go to investigate. Challis had only done one button on his shirt, his belt was unbuckled and his shirt was untucked. They immediately reach the commotion and Challis' shirt is buttoned to the collar, tucked into his pants neatly and his belt is buckled.
(at around 13 mins) Challis goes to take a nap after tending to Harry Grimbridge and helping himself to a cookie from the refrigerator in the Doctors' Lounge at the hospital. The only light left on was the one on the desk in the room. When the sound of Nurse Agnes' screams awaken him and he leaves to investigate there is a two-bulb, standing lamp illuminating the room in the corner where the refrigerator should be.
After Challis fills out the Rose of Shannon motel registry and heads back to his and Ellie's newly reserved room, he passes one of the other motel rooms and the door is shut. When Little Buddy Kupfer's bike falls off his family's RV and Buddy Sr. begins to thank Challis, the same shut door he just passed is now open.
Challis is caught in the factory and struggles with the suited man who attacks him, getting the upper hand and managing to punch through his stomach. When Challis pulls the wiring out of his torso, the man's jacket is buttoned. In the next shot when the orange fluid trickles out of his mouth, his jacket is unbuttoned.
When Challis is caught in the factory he is flipped over a table and lands hard on some boxed inventory, crushing one of them. When he gets up and starts punching his attacker the box is no longer damaged.
(at around 1h 7 mins) When Cochran gives Challis a demonstration of the Halloween mask on the Kupfers while they are in test room A, the closed circuit TV behind them in the final processing area shows Betty walking to the couch and resting her purse on the end while Little Buddy is pacing the room. in the next shot inside the room, she is already sitting on the couch and has one of the fake pieces of fruit in her hand. On the closed circuit TV, Betty leans into Buddy Sr. as the special Silver Shamrock commercial starts. When the next scene cuts into the test room, Betty is resting her chin in her hand and the shot plays out seconds later. As little Buddy is bent over the TV staring at the "Magic Pumpkin" with the mask on, Betty mentions how everything "is a big joke". When the scene cuts back to Cochran, Challis and the robot humanoids again, the closed circuit TV shows little Buddy standing up straight with the mask in his hands.
(at around 57 mins) Dan uses a pay phone inside a phone booth. The bright orange label at the top of the pay phone changes completely from one shot to the next, indicating that two separate phone booths were used for that scene.
(at around 1h 17 mins) As Dr. Challis is confined to a chair in a small room, Cochran gives a lengthy speech explaining the reason for his sinister plans. Before he exits Cochran proceeds to put one of the masks on Challis' head and as he turns on a TV for him in anticipation of "The Big Giveaway", the digital clock above the door states the time as 7:47. 29 seconds after that Challis looks over at the clock and it now 7:49. He spends the next 10 seconds trying to shuffle to the TV and when he looks at the clock again, it is now 7:50.
(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.
(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.
(at around 1h 17 mins) The scenes in the movie Halloween 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.
(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.
(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.
(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 24 mins) When Dr. Challis calls his ex-wife on Friday the 29th to tell her that he can't pick up the kids the next day to spend time with him, he promises to take them trick-or-treating instead. He also says he'll call her Monday morning- but Halloween falls in two days, a Sunday.
(at around 56 mins) The actor on the far left playing one of the five robots that begins to pursue Challis after Ellie's disappearance has a delayed reaction and stumbles sideways as everyone else steps forward.
(at around 1h 19 mins) Challis uses a shard of the broken TV monitor to free himself from captivity in the holding cell. He is trying to cut one part of the strap but it is another part that he lets go, already separated and each end hidden by his fingertips and under his left arm respectively.
(at around 37 mins) When Marge Guttman and Ellie Grimbridge part ways in front of the motel, Marge walks to the front door of her room and pushes the key into the left side. From the camera's POV inside the room the door should've opened from the right- but the door opened left and Marge walked in.
When Cochran gives Dr. Challis a "demonstration" of his plans on the Kupfers in the final admittance area, there is a digital clock display above the wavelength TV that shows the current time in each time zone. If Cochran pursued the giveaway as each time zone struck 9 o'clock, the public at large would've realized his plans after it occurred 1-3 hours earlier Eastern time. If Cochran's plan is supposed to carry out simultaneously from coast to coast after the Big Giveaway at 9pm Pacific time then it would be 10pm Mountain, 11pm Central and midnight Eastern time respectively- far too late for any children to be awake for the sacrifices to occur.
The bathroom of the motel room that Challis and Ellie book for the night of October 29th has blue paint. When Challis is cornered by the suited men the next evening the paint from the inside is still blue but a different color once he jumps out of the bathroom window.
When the hospital assassin sets himself on fire, the shot of the exterior of his car before it explodes clearly shows the driver's-side doors have been removed and then balanced back in place to allow them to blow off easily.
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.