Near the end of the movie, when The Shape grabs Karen's ankle, Allyson tries to stab him with a blood-stained kitchen knife. Half of a second later, the knife has disappeared, to reappear on the floor later on.
As Allyson is walking with her friends along a sidewalk discussing her grandmother they pass a lawn decoration that is a tombstone with RIP. There is an edit to a wider shot during the same sentence, but now they are passing by a plastic skull positioned in front of a tree that had not been there.
When Dr. Sartain lies on the floor after he is being pulled out of the van by Michael his head is smashed to pulp by Michael his foot. When the cops investigate his body his head is more or less recomposed back together.
At around 86 minutes in the film, when Michael Myers has smashed the window pane in the door and is holding Laurie Strode a foot in the air trying to choke her, she manages with her right hand to bring a sawed-off shotgun up from her right side of her body raising it towards Michael Myer's arm that's gripping her. When she shoots the gun, his left hand is free rather than still strangling her. After his hand is blown apart, only then she drops to the ground. Had his hand been free in reality, she would have dropped to ground well before this moment.
When Aaron is pumping gas at the gas station, the pump is not actually pumping any fuel as shown by the display being blank. When he finishes pumping gas later, the pump shows the gallons and cost on the display like normal.
When Sheriff Hawkins enter the house on the domestic disturbance call, he yells out "Haddonfield Sheriff's Department". The patch on his jacket says "Warren County Sheriff". Haddonfield is an incorporated municipality and would not have a county sheriff patrolling but rather a police chief, as sheriffs are county level law enforcement and the municipal police would be the Haddonfield Police Dept.
In the asylum courtyard, the patients are chained to stationary cinder blocks. The safety perimeter around each patient is marked with a large yellow square - to match the checkerboard ground-cover. As chains can move 360-degrees, the safety area 'should' be a large yellow circle. ... Also, the cinder blocks aren't even positioned in the center of the safety squares.
When Michael Myers walks around Laurie's house, the view from the basement shows gaps, dust and even his footsteps through the floor. Earlier the floor was shown to be consisting of square floor tiles without gaps.
The secret door to their "hidden" basement. Opened slower in the earlier part of the movie. (Like a vault door, even sounded heavy as if it was sliding on stone) Once the action starts it opens much faster, not as noisy and even Michael can rip it up off the floor.
Near the end of the film the two police officers guarding Laurie's house are requesting backup after finding a murdered police man (= the doctor) near the police vehicle, which stopped a short distance from them. After the climax, with Laurie's house now on fire, Laurie, her daughter and her granddaughter hitch a ride on a pick up truck, yet backup is nowhere to be seen.
The goof items below may give away important plot points.
Incorrectly regarded as goofs
At one point, Laurie Strode mentions that Michael Myers killed five people in 1978. That would be incorrect because he killed only four people which includes the truck driver, Annie, Linda, and Bob. However, during this scene, there is no mention of the year 1978. Laurie simply says that Michael killed 5 people, and this is including his sister Judith.
Michael murders Ray outside Laurie's house near the end of the film and leaves the body where it is. A short time later, however, Laurie finds Ray's corpse in an upstairs closet despite Michael never having an opportunity to relocate it there.