When Harold, the man in the clown mask, gets the knife from Michael, he starts to tell the details of Michael's killing spree in the first two movies. He says that he killed three nurses and one paramedic, the same night, when in fact the body count was higher. It was four nurses, one being an aide, the doctor, a security guard and a marshall.
In 'Halloween 4: The Return of Michael Myers', a doctor is looking over Michael's medical information. Michael's name is spelled as 'Michael M. Myers'. However, the mental patient at the beginning of this movie states that Michael's middle name is 'Adrian'.
At the house party, the guest in the red coat is in the room when everyone hears the scream and they have to turn down the volume. The next scene at the party shows the guy in the red coat walking in and asking what's going on.
When at his house, the first crew member Michael kicks with a camera. He puts that camera on the ground, tripod closed and laying down. When the people first walk into the house they switch to a camera view that's up the stairs and you can see that the camera that was laying on the floor is now propped up against the wall.
When you see Sara riding her moped up on the road on the way to the meeting with Freddy the sky is blue and dark but when you see her enter the door to the room you can see in the background sunlight coming in from the windows onto the walls.
After Sara receives a message saying that Michael Myers is still alive on her palm pilot Michael Myers stabs Freddy in his left shoulder but in the close-ups of the blade in his shoulder it is in his right shoulder.
In the scene when Sara is looking through the bedroom closet the mannequin with the mask on falls on her and the head falls off but when Rudy comes in and lifts the mannequin back up the head is attached onto it.
When Deckard is at the party he goes up to the opening doors and tries to sneak in. Inside you can just see a lamp and a few bronze pots on the floor but when he enters the room you can see lots and lots of furniture and objects in the room.
In the scene after the fake skeleton breaks through the wall Donna puts her mahogany colored top back on. In a few seconds she kneels down to pick up her camera equipment from the floor without anything with her, but seconds later, when she grabs her camera gear, she has the small camera attached to her ear.
At the start just after Willy sees his partner's head in the washing machine, he gets his throat cut and there is blood all over the knife. Later on when Michael is approaching Laurie's room, his knife just has blood on the tip, only halfway up the blade.
Halfway through the film, we see Michael Myers walk into a room immediately followed by another Myers. Both are dressed nearly identically in a blue jumpsuit. But as soon as the first one turns around and we learn it's actually Freddie, the shot changes to Michael and back to him and he's wearing his jean jacket, seen in the rest of the movie.
When Michael Myers is holding the tripod to impale the crewman, the length of the sharpened end changes in length and shape numerous times. First, it's wider and smaller. Then it's slightly longer and narrower. And when it's finally through the victim, it's long enough to go through the neck and into the adjacent wall.
Near the end of the film, Deckard is communicating to Sara's hand-held device where Michael Myers is in the house. When Deckard tells Sara to "GO NOW!", the text has changed from small digital text to large bold font and the screen has changed from green to white, similar to a large PC monitor.
Donna (the redhead) hastily dons her sweater, sans bra, after her interrupted interlude with Jim in Michael's dungeon. Yet, seconds later, when impaled by the gate spike, she is obviously wearing her bra.
At one point in the garage scene, Michael is lying down in a puddle of blood. However, as the camera shoots Michael's back as he approaches Sara (after he gets up), there is no fresh blood (if any at all) on the back of his suit.
At the beginning of the movie, Harold, the guy in the clown mask says that Michael killed four high school students in 1998 at the academy where Laurie worked. Actually, he only killed three people at the academy; two were students and one was a teacher. Other victims in that film were killed outside of the academy.
When Michael Myers enters Harold's room at the institution and hands over the knife Harold says, "Michael Myers". If you look carefully on the wall on his right you can see shadows of camera equipment and wiring.
At the beginning, when Michael kills the first security guard in the basement, just before he dispatches the second one, you can see a crew member's arm reflected in a window hitting the light shade to keep it swinging.
When the rope pulls Micheal off the ground, he drops his knife. When Laurie looks at the knife it has little dots on the handle, but when she picks the knife up the dots are gone and the blood is different.
In the beginning when they have the flash-back to H20, Michael knocks out the paramedic. As he is strangling him, the mask he is wearing is not the same mask from H20. It is the mask from Resurrection.
The goof item below may give away important plot points.
When Jen is decapitated, her severed head lands in front of the stairs. A few shots later when Sara returns to the same room, Jen's severed head has disappeared. However it can still be seen on one of the TV screens later in the movie.