In the scene where Ralph comes out of the pantry, Ned comes running in with a hamburger in his hand. The hamburgers aren't even made yet. In fact they are being made in the next scene.
In the close up shots of Marcie's face while she and Jack have sex, Jack's hand can be seen near her neck. In the shots farther out, Jack's hands are down by her waist.
Brenda says she rolled doubled sixes while playing Monopoly, but you can see she actually rolled a one and a two.
Brenda walks to the bathroom while its pouring outside. Yet when she enters the bathroom her rain coat doesn't appear to be dripping water, and her legs are not wet at all. That's impossible.
During the Monopoly game you can see a red paper on the piano in the background. Later after Brenda has been killed, and Alice hears her scream, the red paper is now behind a blue paper. After Alice discovers Bill dead, the red paper is back in its original place.
When Alice wakes up on the couch after Bill has left her socks are white. But two seconds later as she shuffles into the kitchen its obvious that her socks are dark, not white.
Alice runs away from Mrs. Voorhees, and heads into a barn. As she's opening the door a piece of wood leaning up against the side falls over. Later when we see a shot of it the piece of wood is back to its original position.
When Brenda's body is thrown through the window around 1:14, the blood on her chest is on the left side. But later around 1:24 when Alice runs back into the kitchen to hide in the pantry, the blood is on her right side.
After Mrs. Voorhees is hit in the head with a frying pan she falls on the floor slightly on her back, but turned, and her arms up. In the next shot she's fully on her back, and her arms are in a different position.
Alice starts to make some tea while she waits for Bill to come back. She never takes it off the burner, and we don't see her shut it off. But it doesn't appear to be lit later.
In the beginning when Claudette and the counselors are singing and playing guitar, the guitar music continues even after she stops strumming the guitar strings.
In the same scene above, its quite obvious that Claudette is not playing the guitar.
Also in the beginning when the counselors are singing, Claudette's legs go from being crossed to uncrossed.
Claudette's socks are pulled way up when she leaves the cabin, but when she enters the barn, her socks are pulled down.
In the scene where Ned sees the killer heading into the cabin, you can tell that the killer is wearing a black or very dark shiny rain jacket. A few minutes later Marcie and Jack head into that very same cabin. Ned is already dead, and the killer must be under the bed the entire time they are having sex. Yet when the killer’s hand reaches up, its wearing the same flannel shirt from when the killer killed Annie. Not the rain jacket. Later on when Brenda is out in the archery range and the lights come on, you can see the killer is wearing the rain slicker again, of course this time it is raining. Later when the killer is reveled she’s wearing a blue sweater and a black turtleneck.
Kat's Explanation: If you look a little closer on the scene where she is speaking with Alice you can see that she is wearing the flannel shirt under her sweater. Just look at the neckline. The raincoat makes sense for the scene where she kills Brenda, but doesn’t make much sense for when she goes into the cabin and kills Ned. It wasn’t raining then.
Annie says that she hates it when people refer to to children as "kids" because they sound like goats. Yet, earlier in she states that she will be cooking for fifty kids.
Kat's Explanation: She could be referring to them as Kids because that was how Steve referred to them, and she was just repeating what Steve had said.
In the same scene, Annie says that she will be cooking for 50 kids and 10 staff. But there is only Brenda, Annie, Jack, Marcie, Bill, Alice, Steve, and Ned, that makes 8 staff members, and 7 if she doesn't include herself.
When Annie enters the dinner, the storekeeper is leaning against the counter, but when the camera angel changes she isn't leaning on anything and the guy who was sweeping on the other side of the counter is behind her. When the angel returns two shots later, she is leaning again on the counter and the guy is back in his original place.
When the counselors get the tree stump out of the ground, Alice is shown putting down a bucket, but in the next shot she is putting the bucket down again.
When Ned, Jack, and Marcie are driving in the truck, the background changes, back and forth between a country road with many trees, and a main road, with almost no trees.
During Annie's death scene, where she is backing up against the tree, the amount of leaves in her hair keeps changing.
When Steve is talking to Alice about her leaving, while she is hammering, his hands go from being on the gutter, to his arms by his side.
When Alice is freaked out by the snake, she lifts up the sheet, but in the next shot its Marcie who is lifting up the sheet, and Alice is standing behind her.
When the cop is giving Steve a ride, he turns on his lights, but in the next shot the lights aren't on, and then they are back again.
While Jack is smoking a joint both of his arms are in front of him, but in the next shot, when he's being stabbed, his arms are beside him.
Kat's Explanation: This is because Jack's body is that of a dummy. It was made for his death scene.
In the same scene, the killers arm changes shots from the zoomed out shot, to the close up.
When Mrs. Voorhees is running at Alice, Alice swings with the fire poker and hits her, but in the next shot, its like nothing happened. Mrs. Voorhees is running at her again.
When Alice is making tea, on the table behind her is a basket of eggs and a blender, when Alice returns to the kitchen later, after she finds Bill's body, the blender and basket of eggs have switched places.
The movie is supposed to take place in the early summer, but when Jason pulls Alice under water at the end, the leaves on the trees are yellow, orange, and brown, as if it were fall.
Kat's Explanation: This is because this scene was filmed three different times, the last being in the fall.
When the group is playing Monopoly they are going in a certain order - Brenda, Bill, Alice. But in the next shot they are going in a different order - Alice, Brenda, Bill, which is still correct, but then they switch to Brenda, Alice, Bill, which is incorrect.
Annie jumps out of the jeep and lands on her head, and shoulders, yet, her ankle is twisted, because she's limping.
Kat's Explanation: Maybe she twisted it as she was trying to get up.
Bill gets stuck to the door by arrows. It's not possible for those arrows to hold all of his weight on the door.
At the diner Steve says that he has six new counselors, yet its really seven. Alice, Brenda, Marcie, Jack, Ned, Bill, and Annie. He had no idea that Annie was dead.
Kat's Explanation: It's possible that since Annie was not there when he left, he did not include her. But, by this point in time, it is dark outside and Annie should have made it to the camp.
Ned had been dead for awhile, why did it take so long for his blood to drip on Jack?
Kat's Explanation: It had to go through the mattress.
After Brenda's dead and she is thrown through the window, her head moves.
Kat's Explanation: People can still move (twitch) when they are dead.
Its been raining all night, and the canoe has been outside all night, how come the canoe doesn't have a lot of water in it?
Alice swings the machete to the left side of Pamela's neck, but Pamela's neck splits on the right.
When Alice discovers Bill's body hanging on the door, his right eye twitches.
Kat's Explanation: This is because the makeup irritated the actors eye.
Alice is hunting for bullets in the filing cabinet, the lock and chain is just looped through the handles, she would have been able to pull out the doors. The chain gave her enough leeway.
Brenda already has a sink in her cabin, why in the world would she walk out in the rain, to only brush her teeth and wash her face?
Kat's Explanation: Maybe the one in her cabin didn't work. Its possible she went in there to use the toilet, but we don't see her do this, and we hear her enter the room, and leave the room holding all her stuff.
The nurse to the left, in the hospital glances at the camera for a second.
After Brenda is thrown through the window Alice runs out of the room and gets the door open. This doesn't give Mrs. Voorhees enough time to be behind the cabin, throw the body, run around front and get in the jeep. Plus you would think Alice would have heard Mrs. Voorhees go around the cabin.
Bill is obviously not playing guitar. The actor playing Bill, Harry Crosby, has stated that he did in fact play the guitar. Perhaps they dubbed a song over it as his hands just are not moving properly.
When does Steve attach the trailer to the Jeep?
Kat's Explanation: We haven't seen him in awhile, so he put it on off screen.
The cop gets a call and tells the dispatcher that he'll be there in 15 minutes, then turns his lights on. In the next shot he's turning them on again.
When Marcie and Jack are done having sex, Marcie gets out of bed and goes across the room to put on her rain jacket. She goes back to the bed and gives Jack a kiss before heading off to the bathroom. There is a lit candle on the table beside Jack. Wouldn't the candlelight illuminate the area of the bunk bed so she would see Ned's dead body?
The arms that reach up, right after Mrs. Voorhees' head is chopped off, are obviously man's. You can see the hair on the wrist. Its proven to be Tom Savini's FX assistant, Taso Stavrakis.
You can tell that Alice isn't really holding the machete which cuts Pamela's head off.
When the arrow is shoved into Jack's neck, you can see the line, of where the fake body and head connect.
Mrs. Voorhees' head falls off before the machete even comes in contact with her neck.
On the original VHS box, Laurie Bartram's name is misspelled as Laurie Bartham.
When the axe is discovered on the bed, only one side has blood on it, why would the other side be totally clean?
Outside the Diner its raining pretty hard, but the rain looks very fake.
Kat's Explanation: That's because it is fake.
The lightning on Marcie and Jack while they are outside, looks to be fake, however in interviews its said that it was a real storm.
When Marcie goes to look behind the shower curtain, a large black line runs up the left side of the film.
While Brenda is brushing her teeth the light goes from moving to not moving, to moving again after she leaves.
Kat's Explanation: Its possible that it is supposed to move after she leaves. Maybe it was the killer watching her.
Brenda goes outside because she hears a child's voice calling for help, with the pitch of the voice, there is no way that Mrs. Voorhees could have called out like that. Especially when we hear what her "child" voice sound like.
Kat's Explanation: Some people even suggest that perhaps Jason was around calling out to Brenda. However, it was in fact Betsy Palmer making the child's voice.
Ralph put his bike against a tree outside the cabin, didn't anyone notice that it was out there?
Steve tells Alice to have Brenda help paint but she never goes to help Bill Paint.
Steve tells Brenda to finish setting up the archery range after lunch, but she does it after he leaves - before lunch.
Kat's Explanation: Maybe she didn't want to wait till after lunch.
The cop on the motorcycle goes down the hill, as he slows down he upshifts to a higher gear as he's slowing down. Then all of the sudden the sound his engine makes is obviously (to those that know motorcycles) the sound of a completely different engine.
Kat's Explanation: I have no idea why they would have changed it, unless the original recording of the sound didn't sound right.
Mrs. Voorhees has proven to be a brutal killer. She's killed all these counselors without playing with them first. So why in the world does she throw string at Alice, when she could have thrown anything else. And why does she only slap her?
Kat's Explanation: It's very likely that Mrs. Voorhees wanted to taker her time in killing Alice since Mrs. Voorhees knew that Alice was the only one left at camp.