(at around 9 mins) The Elrods are watching Night of the Living Dead and we hear Johnny's dialogue, "They're coming to get you, Barbra" overlapped by the movie's music score. In the actual movie, the music score doesn't begin until after Johnny is attacked and Barbra begins fleeing.
When Jimmy is telling Laurie about who attacked her that night and he says "It's on right now" (referring to the media coverage), the shot from behind shows that his mouth isn't moving and he is sitting very still with his hand on her waist.
In the ambulance, Jimmy tells Laurie they're taking her to "the clinic". The radio announcer also reveals that Laurie was taken "across town to Haddonfield Memorial Clinic". However, the jackets that the medics wear, as well as all the signs at the hospital, indicate that it's actually Haddonfield Memorial Hospital.
(at around 53 mins) Dr. Loomis and the policemen find the word "Samhain" written on the blackboard at the elementary school, and Loomis claims he knows the meaning of the word, delivering a speech about the "festival of Samhain". However, he pronounces the word literally, which is incorrect. It's actually pronounced "sow-in".
(at around 40 mins) Dr. Loomis says that Michael is 21 years-old, yet when the credits appear at the end his age is given as 23. (This was a mistake in the credits for the original Halloween film. As Michael was 6 years old when he killed his sister Judith, in 1963, he would have been 21 15 years later in 1978.)
(2 minutes into the movie) When Laurie takes off Michael's mask in the beginning, Michael does not leave the mask off long enough so that we could see his deformed eye like we could see in the previous movie.
(at around 2 mins) In Halloween, Michael falls off the back balcony of the Doyle house, however in this film he falls off the front balcony. The balcony in the first movie is a covered balcony, the balcony in this film is not.
At the end of Halloween Michael Myers was shot by Dr. Loomis six times, and in this movie it's said repeatedly that he was shot six times. In the recreation of that scene at the beginning, however, it's seven. Additionally, seven shots would not have been possible from Loomis' six-shooter.
It's supposed to be a continuation of the same night from Halloween, yet the landscaping for the Wallace and Doyle houses has changed considerably. Also, the jack-o'-lantern on the front porch of the Doyle House has changed completely.
Stuntman Dick Warlock's eyes are very noticeably blue throughout the movie, yet Michael's character is supposed to have "the blackest eyes, the devils eyes." Tony Moran's eyes in the original Halloween's unmasked shot appeared brown.
(at around 1h 26 mins) After the explosion and Michael is walking on fire, he is right next to the burning stretcher. The next time we see him, he is about eight feet behind the stretcher walking towards her again.
(at around 8 mins) When Mrs. Elrod asks her unknowingly sleeping husband what he wants on his sandwich and Michael Myers is stalking them from outside, she approaches the doorway between the kitchen and the living room. If you look carefully behind her as she moves there is nothing on the windowsill, although there was a pumpkin present when Michael walked up to the house. The jack-o'-lantern in their window disappears whenever the camera is facing everyone and reappears behind them from Michael's vantage point.
At the end of Halloween, when Loomis shot Michael off the balcony afterwards he looked down from the balcony to see Michael wasn't there and he wasn't surprised by this, yet at the beginning of this film, Loomis goes down stairs after shooting Michael and is shocked to see him gone.
(at around 22 mins) Just before the sheriff closes his dead daughter's eyes outside the house, he pulls the blanket covering her down to her chest, just after he closes her eyes the blanket is pulled right up to her chin.
(at around 15 mins) The flashing lights on the front of Bud and Jimmy's ambulance change places as they pull into the hospital driveway. They go from either side of the hood to the front of the bumper.
(at around 14 mins) When Gary and his mother arrive at the hospital, the ambulance containing Laurie can be seen driving up in the background. In the next shot, the ambulance enters the parking lot and drives up behind them again.
(at around 1h 17 mins) Laurie is on the ground beside Jimmy's car outside the hospital when she sees the Marshal's car pull up. When Loomis tells the Marshal he ought to handcuff him to the wheel, Jimmy's car is visible in the distance but Laurie is gone.
When the first close-up of the ambulance is seen driving up to the hospital, you can see on its front bumper that it does not have red flashing lights. However, in the next shot, it has 2 red flashing lights mounted on the front bumper.
(at around 13 mins) When Laurie is wheeled into the ambulance, the blanket on the stretcher isn't covering her feet, but when the paramedics lift her out of the ambulance, the blanket covers the whole of her feet up.
(at around 9 mins) Near the beginning of the film, when Michael walks into the Elrod house, Mrs. Elrod is seen standing in front of the door from the kitchen to the living room, and when Michael picks up the knife, she's seen standing to the side inside the living room.
The movie ended in the first Halloween film with the sound of Michael Myers breathing heavily and varying shots of where he acted out his reign of terror on Laurie Strode and her friends. One was of the front of the Wallace residence, the misshapen jack-o'-lantern on the porch dimly illuminated. Halloween II is supposed to be a direct continuation of "The Night He Came Home", but when Annie's body is being taken away the pumpkin in front of the house is full and well-lit.
(at around 11 mins) Alice hears Mrs. Elrod scream and calls out asking if her and her husband are "okay". When she returns to her phone call with Sally she holds the receiver in her left hand and places the right one on her hip. In the immediate next shot, Sally asks Alice if she heard about the girl being killed in Haddenfield and the receiver is now in Alice's right hand. Her left one is straight at her side.
Mrs. Elrod is supposed to be paying attention to the television as the news reporter is announcing the murders and Michael Myers' escape, but during the latter's theft of the kitchen knife there is a clear view of the TV and the screen is bouncing out of frequency. You can hear the reporter but the visuals aren't there.
When Sheriff Brackett went to confirm that one of the victims at the Wallace residence was his daughter Annie, the digital watch on his wrist said 12:33am but the scene immediately before this was children trick-o'-treating being pushed aside by Dr. Loomis' when he thought he spotted Michael Myers.
Before the final confrontation between Dr. Loomis, Laurie and Michael Myers, the latter approaches the door of the operating room while the former try to hide. As Michael is looking in the door is solid with a thick window frame and has a deep wooden stain. As Michael begins to succeed in tearing it apart, the door is now flimsy with a faded look to it.
When Michael Myers finally locates Laurie's room and finds her missing, he departs with the door still open. As Nurse Jill returns to the room from tending to another patient right afterward she enters by opening the now-closed door.
Jimmy realizes Laurie isn't responding after speaking to her and rushes to get one of the nurses. Nurse Janet is standing by the jack-o-lantern but when Nurse Jill returns with Jimmy immediately after Janet is not there and already running for Dr. Mixter.
From the beginning of the film up to when the police car drives up to the Doyle residence, the sky is black. When the paramedics take Laurie from the residence to the ambulance the sky is dark blue. For the rest of the film until the morning after Halloween, the sky is dark again.
The film is a direct continuation of the first one but as Laurie is being taken away from the Doyle residence to the ambulance pressure is being applied to the bloody wound on her left shoulder. Michael Myers slashed her on the left upper arm, an area lower than where they are treating.
At the very start of the film, the camera pans down from the top of the trees. The porch lights are on and the front door of the Doyle residence is open. As a direct continuation of the first film we see Laurie instructing the kids and they run for help, the porch lights completely off and the door opened by Tommy. When Dr. Loomis goes to look for Michael the porch light is back on and he runs out of an already open door.
Although Laurie had positioned her pillows in the hospital bed to look like a body before secretly leaving the room, they both change position (with the bedsheet) to the very bottom between the brief time Michael exits and Nurse Jill enters.
When Michael physically breaks down the door to the operating room, he destroys it all the way up to the top of the door frame. After he stabs Dr. Loomis the door is only damaged now to the top of the window frame.
When the Marshal is escorting Nurse Chambers and Dr. Loomis back to Smith's Grove in his vehicle, there is a blinking light in the rear window. When Dr. Loomis shoots out the window the car screeches to a turn and the light is no longer there. As they reach the hospital the light is on and blinking again.
When Dr. Mister plunges the needle into Laurie's arm, the tourniquet that Mrs. Alves applied is knotted on her forearm. As he withdraws it the knot is no longer present and one side is folded over her arm. Mrs. Alves removes the plastic band and tosses it in the tray with the other supplies but when Dr. Mixter administers the dose to make Laurie go under, there is another knotted tourniquet although no one is seen tying a new one on her arm.
As seen in the cut from the first film at the start of the movie, Jamie Lee has a silver band on the fourth finger of her right hand. In the scene where she is being treated upon arrival at the hospital, the ring is now on the fourth finger of her left hand.
As Deputy Hunt and Dr. Loomis are examining the burnt corpse with the dentist at the morgue Deputy Graham walks in, Dr. Loomis' hand seen cupping his mouth. As Deputy Hunt orders Deputy Graham to reopen the search for Michael Myers in the immediate next shot Dr. Loomis only has his finger on his moustache.
(at around 1h 12 mins) When Michael corners Laurie in the elevator, the door slams on his hand and keeps closing. Elevators have rubber guards that cause the door to open again if someone's hand would get caught in the closing.
(at around 1h 21 mins) Marion calls for help on the police car's 2-way radio, then listens to the reply without letting go of the "talk" button on the radio's microphone. You can't listen and receive anything on the radio while the button on the mic is pressed.
The elevator should've never taken Laurie from the basement to the first floor because she didn't make a selection upon entering- all of the elevator buttons are unilluminated as she cowers in the back from Michael and the closing doors. In reality, the elevator car would've stayed dormant on the basement level.
All of the deputies (patrolmen) are credited wrong in their order or numbering. Dick Warlock is credited as Patrolman#3, when he is actually the first one seen, crashing the car into Bennett Tramer. Ken Smolka is credited as Patrolman#1, when he is the second one seen, at the Meyers house and at the morgue. Roger Hampton is credited as Patrolman#2, when he is the third one seen, at the elementary school.
Nurse Jill's name tag has 'R.N.' after her name, which stands for 'Registered Nurse'. Registered Nurses have black stripes on the cap, like Nurse Mrs. Alves. Nurse Jill's cap does not have the black stripes, indicating she is in fact an L.P.N., or Licensed Practice Nurse, not a Registered Nurse.
Considering how immense Haddonfield Memorial Hospital is (it has wings), and the fact that it has an emergency room and surgical ward, it should not have just four nurses, one doctor, and two ambulance drivers staffed for the entire hospital, even if it is the night shift.
At the beginning of the movie, it is mentioned several times (on TV on the radio and in telephone conversation) that 3 teenagers were killed, but then police discovers the bodies only after that - which we learn in the conversation between Sheriff Brackett and his deputy who tells him they have just discovered 3 bodies "in the house across the Doyle's house", and that his daughter was one of them.
(at around 1h 15 mins) When Loomis and the nurse are in the deputy's car, Loomis shoots out the window and the car makes a hasty reverse turn. On the road, there are at least 2 identical swerve marks. This would indicate the scene had been shot multiple times.
(at around 18 mins) When Dr. Loomis and Sheriff Brackett are in the car, one of Brackett's police officers call in on the radio. Many times throughout their conversation, the man on the radio talks before Brackett lets go of the button.
Haddonfield is portrayed as a small Illinois town in the film- when taken to and admitted in the hospital Jimmy, Dr. Mixter, and Nurse Jill called Laurie personally by name. Bud also figured out that her father ("her old man") owned the realty business. However, it took Sheriff Brackett's peers an hour to inform him that his daughter Annie was one of the victims at the Wallace residence.
The goof items below may give away important plot points.
(at around 1h 24 mins) After Michael is shot in the eyes and blinded, he starts swinging the scalpel wildly at Loomis and Laurie when they start releasing the gas. As Laurie flees before Loomis lights the place up, the sound of Michael swinging the scalpel is out of synch by around half a second when he actually does swing it.
(at around 1h 6 mins) After finding the head nurse dead, Jimmy turns to leave the room and slips in the blood and gets it all over his face and jacket. When we next see him in the car (at around 1h 16 mins), there is no blood on his face or clothes.
(at around 1h 19 mins) Near the film's climax, the Marshal is shown walking down a hallway by a soda machine and reacting to Laurie's screaming. Several minutes (and many screams) later, Michael crashes through the door and the Marshal is still running down the hallway by the soda machine.
(at around 19 mins) When the police car hits Ben Tramer and pins him against the van, there a shot where his upper half falls and his head hits the hood of the police car while on fire. In the very next shot, the body is instead shown upright while in flames.
(at around 1h 23 mins) After Dr. Loomis tries to shoot Michael Myers with the empty pistol and is stabbed with the surgical knife there is thick, dripping blood all over the instrument. Dr. Loomis then covers his wound with his hands and falls beside one of the hospital beds. Once Laurie has shot Michael's eyes out Dr. Loomis pulls the gas tank tubes out of the wall with one hand and holds a lighter in the other hand to ignite the fumes, but his hands are completely clean.
(at around 1h 25 mins) Doctor Loomis fills the room with flammable gas. He then lights a lighter that makes a flame, hesitates, and then ignites the gas in an explosion. The same sparks that ignited the lighter fuel would have ignited the flammable gas simultaneously.
(at around 52 mins) Assuming the hot tub water was hot enough to scald Nurse Karen's face, as depicted, then it could be assumed that Michael Myers' hand, which was also clearly seen under the water, would show similar burn marks; it does not. In reality the water would not be hot enough to burn flesh at 125 degrees Fahrenheit.
(at around 1h 26 mins) At the end, as Michael emerges from the explosion, his body embraced in flames, one can easily tell that the clothing the actor is wearing is much thicker all over and now includes gloves that the character Michael was not wearing before.