Factual error: The "big giveaway" that happens at 9pm across the whole country would have to happen at least five different consecutive times due to time zones. The "big giveaway" in the most eastern time zone would be successful, but further west it would be earlier than 9pm.
Continuity mistake: 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.
Continuity mistake: During the scene where Dr. Challis is at the bar and Ellie walks in to find him, look carefully as Ellie approaches Challis. In the long shot Challis takes a drag from his cigarette and is exhaling the smoke just as Ellie walks up, but a split second later, Challis is not blowing any smoke out. In fact, there isn't even any smoke around where he would have exhaled.
Plot hole: 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, why would the liquor store still be open?
Continuity mistake: Ellie and Rafferty stand outside the hotel while Challis 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.
Continuity mistake: Right after Ellie was kidnapped, Dr. Challis runs to a pay phone and dials the Operator to get help, and begins to state, "Operator." and gets a recording. Not only does he get a recording (which would be impossible in the real world), but the recording itself states, "Your call cannot be completed as dialed. Please check the number and dial again, or, call your local operator." Problem is - he was dialing the operator.
Continuity mistake: Towards the end of the movie, Challis's shirt goes from untucked to tucked several times between shots.
Continuity mistake: 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.
Continuity mistake: During the night scene where Ellie is walking back to the hotel room, a wide camera shot of the entire parking lot shows no cars (not even theirs) adjacent to the walkway. A second later, as Ellie gets closer to her hotel room, Ms. Gutman's car is now there and honks the horn at her.
Factual error: Right after the assassin kills Harry Grimbridge in the hospital, he walks out to his car to set himself on fire. We see him pour gasoline on his face and chest, flick the lighter, and IMMEDIATELY the car blows up. This is near impossible. In order for the car to blow up, fire would have to reach the fuel tank or lines. The body would have to sit burning in the car for at least a little while in order for the fire to properly spread to certain areas where the car could then blow up. Secondly, there have been many real-life instances where bodies have been set afire inside cars and the car has never 'blown up'.
Continuity mistake: After the old man is killed in the hospital, the killer gets into his car and sets himself on fire with gasoline. The car then explodes, causing parts of the car to shoot all over the parking lot. These car parts are on fire and surround the car, but a few seconds later they have disappeared.
Factual error: At the very end of the movie, we see Dr. Challis rushing to the gas station phone with seconds to spare, calling the "networks" to try and prevent the deadly commercial from airing. Almost impossible for several reasons. First off, Challis only calls one network (we don't know which one) and yet they seem to pull the commercial from two of the three main stations, which is unlikely (not all networks are affiliated). Secondly, without any special credentials, it is near impossible to get something pulled off the air, even in a legitimate emergency. Third and lastly, it seems even more impossible that given all of the above, any of this could have been done in under a minute's time.
Revealing mistake: During the part when the day reaches October 31st, various scenes show children and happenings with their Halloween masks throughout various portions of the country getting ready for the big give-away. Included with this is the scene showing Dr. Challis' two kids again sitting in front of the television with their masks on, but they are wearing the same exact clothes they had on when he had stopped over at their house 8 days earlier. This scene was obviously cutting-room floor from that previous scene.
Continuity mistake: Dr. Challis enters his ex-wife's home with his coat over his head from the rainstorm. His hair is slightly wet, but the coat itself appears to not have a drop of rain on it.
Factual error: The story takes time the last week of October. When Challis and Ellie are on their way to Santa Mira, the trees have green leaves. Even in California, this isn't usual.
Continuity mistake: 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.
Revealing mistake: During the scene right after Dr. Challis breaks into Silver Shamrock and comes upon the "old lady" (robot) in the chair knitting, it is easy to tell if you look at the movements of the hands before Challis grabs her, it is indeed a real person (probably a stunt-person) doing the knitting, and the hands are real. Then, when Challis grabs her and she comes apart, the hands and other body parts on the floor are now all plastic with gears.
Factual error: During the scene where Cochran had Dr. Challis tied up in the chair with the mask on in front of the television, Dr. Challis eventually moves the chair close enough to the television, where he literally kicks right through it. This seems highly inaccurate on several levels. First off, Dr. Challis' upper legs were still tied down which would have limited his strength considerably. Second but most importantly, the television itself nor the cart it was on weren't "anchored down" or "bolted down" in any fashion bolstering any leverage for him to be able to put his feet straight through the television. In reality, his feet should have just pushed the tv off the cart and onto the floor.
Revealing mistake: In Buddy Kupfer's death scene as "little" Buddy falls to the floor and the creatures exit his mask, his hand moves across the floor in one shot after several creatures have already exited the mask (even the rattlesnake wouldn't have the power to do that) and he is obviously dead.