Judging by the cars, music and fashions, the early part of the film takes place in the late 1970s (probably 1978, as a nod to the release year of the original Halloween); in the latter part of the film, Dr. Sam Loomis mentions that 17 years have passed since he met Michael Myers, which would make the time period circa-1995. However, technology (such as cars and cell phones) and fashions in this part of the film clearly place it in the mid-2000s.
(at around 33 mins) No asylum where a character is imprisoned as criminally insane would give inmates metal forks. They would be replaced by plastic cutlery, for exactly the reasons that they end up being used here - fear of being used as weapons to attack staff or other inmates.
The title card after Deborah's suicide says Michael's escape takes place "Fifteen Years Later." However, the very next night, Dr. Loomis and Sheriff Brackett both refer to the time since her suicide as having been seventeen years.
(at around 48 mins) When Michael and Big Joe Grizzly fight in the bathroom stall, Michael slams him into the stall wall which collapses into the other stall. In the next scene, before Myers takes the overalls, you can see that the stall wall is back up and only slightly dented.
(at around 1h 24 mins) When Michael Myers breaks through the bathroom door, an officer shoots him in the left shoulder. They next show Michael stabbing the officer, when he turns to the camera, the bullet hole is gone.
(at around 46 mins) When Big Joe Grizzly (Ken Foree) climbs out of his truck the wash attendant lifts the hood then goes for a hose. When the camera switches to where Ken's character is walking, you can see in the background the attendant lifts the hood and goes for the hose again.
(at around 53 mins) Laurie is dropping off the envelope for her father and you see her pretend that someone is in there taking the envelope from her and then you see her drop it through the mail slot. She then turns to talk to the little boy but when she turns back to the house you see her drop the envelope through the mail slot again.
Laurie calls the police from Lindsay's house, 1987 Winchester Drive, and gives that address. Yet when the police arrive they immediately and conveniently go to 1960 Winchester Drive, Tommy's house - where Laurie is now hiding - despite it being on the other side of the street and several houses down. Some minutes later, responding to the same call, Dr. Loomis and Sheriff Brackett correctly arrive at Lindsay's house.
Just after the rape scene, Michael Myers is attacked in the back by a baton by one of the hospital workers - it leaves a very noticeable long baton-shaped dent in his back, making it very obvious that he's wearing a badly padded out bodysuit.
(at around 30 mins) During the winter visit to the jail by Dr. Loomis; you can see a deciduous tree in the upper left that has leaves that are as green as they are in the summer, yet there is six inches of snow on the ground.
The goof item below may give away important plot points.
(at around 1h 35 mins) When Michael attacks Dr. Loomis, we clearly see blood coming down Loomis' face, but when Michael drags him into the house afterwards, we get several views of Dr. Loomis' face, and it has no blood on it.
(at around 1h 7 mins) During their struggle when Michael kills Laurie's adoptive mother (Dee Wallace), the phone gets knocked off the hook. Yet when the sheriff calls the house later to inform them of the danger, he gets a ring and an answering machine.
(at around 1h 40 mins) After Laurie and Michael fall out of the window, she picks up the revolver and as she pulls the trigger it clicks on three empty chambers. This would not be the case if Dr. Loomis had already fired the gun. Laurie would have not hit on the three empty chambers but on the three loaded chambers.