Because of all the re-shoots after principal photography had finished, hairstyles change throughout the film (especially Sara's)
(at around 44 mins) 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.
In the garage Sara falls down and a piece of electronic equipment lands on her. She acts like it's so heavy that it has her unable to move. Yet when Freddy comes to her aid, right before he lifts it off, Sara starts to get up on her own.
(at around 1h 3 mins) The blond girl has very thick black roots. She is decapitated. When Michael throws her head down the stairs, she is perfectly blonde, no roots.
(at around 11 mins) When Laurie hits Michael over the head with the lamp, when he walks through the door the lamp smashes. Then when Laurie drops the lamp and runs out of the room the lamp is not broken and is still lit.
(at around 19 mins) The school email is listed as ".org", but it should be ".edu".
(at around 12 mins) When the rope pulls Michael 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.
Michael cuts Willie's throat and drops him. When he hits the floor, he is dead and his eyes are wide open. But if one has their throat cut, one does NOT die immediately. Rather, if one's throat is cut, one will bleed to death. This process will take a couple of minutes for a person to bleed out.
(at around 1h 13 mins) It's mentioned in this movie that Michael Myers lived under his house for 20 years, contradicting everything that happened in Halloween 4: The Return of Michael Myers (1988), Halloween 5: The Revenge of Michael Myers (1989), and Halloween: The Curse of Michael Myers (1995); however, Halloween H20: 20 Years Later (1998) worked on a continuity where the later sequels were ignored, continuing instead from the end of Halloween II (1981). This means, that in this continuity, Michael Myers could have been living under his house for two decades.
(at around 3 mins) 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.
(at around 58 mins) After Michael kills Donna by the gate post, her head is still moving a little after she is dead.
(at around 1h 13 mins) When Michael is hanged, there's an upward shot showing his body go limp. The roof of the stage can clearly be seen - rafters bathed in green light.
(at around 1h 16 mins) Obvious stuntwoman in the garage when Sara Moyer slips in the puddle of blood and falls flat on her back.
(at around 41 mins) After the hallway camera cuts out, it eventually comes back on the computer screen that Deckard is looking at. If you look at the landing and the stairway, the picture is reversed.
When Freddy is watching Fist Of Fury he hears the doorbell and says something as he turns his head, but his mouth doesn't move.
Take note of when Michael Myers brandishes his knife. Any time the knife is shown, an audible, metallic 'SHHHING!' is heard, as though the weapon is being pulled out of a sheath or scabbard. Even when the knife is being held still, the sound effect is still heard. It gets to the point of being almost comical.
(at around 24 mins) When Freddie is watching Fist of Fury (1972) he hears the doorbell and says something as he turns his head, but his mouth doesn't move.
(at around 42 mins) When Myles is at the party in the office checking out the web broadcast, a guy and a girl bust in and intend to have sex there. What we hear the guy say doesn't match up with his mouth movements.
(at around 57 mins) When Donna runs into Michael in the sewer, she says "Oh, shit," but a dubbed over scream is heard instead.
(at around 16 mins) After Michael hands Harold the knife, the camera is reflected in the window as it leaves the room.
(at around 15 mins) 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 around 1h 21 mins) Just after the garage has burnt down the police arrive and if you look very closely on the windshield of the first you can faintly see a camera crane lowering down.
(at around 9 mins) 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.
(at around 28 mins) When you see Freddie's black Voyager parked outside, a green Jeep passes by the car. If you look carefully at the reflection there is a large, high ceiling of a warehouse stage set.
(at around 21 mins) When Sara is riding up to the 2400 Court Motel to see Freddie, the entire city of Vancouver can be seen in the distance behind her.
Michael's first scene in the movie shows him wearing the mask, but it's never disclosed where he got it.
(at around 16 mins) 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.
(at around 7 mins) When Harold - the patient who knows all about serial killers - is taken in he says John Wayne Gacy died May 7th 1998. John Wayne Gacy was put to death on May 10, 1994.
It is revealed in this movie that in Halloween H20, Michael crushed a paramedic's throat and switched clothes with him (including the mask), which caused the appearance of Michael being killed ... until he is unmasked. In the paramedic's uniform, Michael was able to roam around the scene where the paramedic was killed, and no one noticed. This seems especially odd since the story took place in a smaller community where people would have been familiar with certain other people, including first responders.
(at around 20 mins) When Sara's first e-mail arrives in Myles' inbox, Scott reads its content aloud as: "Deckard, you'll never believe what I got myself into". However, the e-mail actually reads: "Deckard, you're not going to believe what I've gotten myself into."