When the group of teens come across Mrs. Voorhees' headstone, it reads that she died in 1979. In the beginning of Pt. 1, the film states that it's the present date, which would have meant that Mrs. Voorhees died, in 1980.
Kat's Explanation: The film was shot in 1979, but released in 1980.
In Part III, Jason was stabbed in the leg and his hand, yet in Part IV (which takes place the next day), there is no blood stain nor cut in his pants or his leg, and no wound on his hand.
In the other films Jason had normal fingernails, in this film they are black and its only been a few days between parts 2-4.
Tommy went from having brown thick hair to thin light blond to wavy dirty blond through Parts 4, 5, and 6.
Kat's Explanation: In the novel they explain that away by saying that 4-6 happens over a course of 10 years.
Part 3 ends and part 4 starts that same night Jason's face could not have changed that much in just one day!
Rob tells Trish and Tommy that he's there hunting bear. Tommy says there are no bears around, but just a few days before Paul (Pt2) tells everyone to watch out for the bears.
In Part 1 Mrs. Voorhees states that Jason wasn't a good swimmer. This we know since he drown. So why does he swim out to kill Sam?
When Trish and Rob get back to the house, they end up breaking one of the windows in the door so they can get in. A few minutes later they tell Tommy to lock the doors while they go check out the house next door. Jason could have just put his arm through the window like they did.
When Trish comes back from the house next door, after Rob has been killed, she asks Tommy to get her a hammer and nails. She yells “Tommy! Hurry!” a few seconds later. It sounds almost like she is saying “Corey! Hurry!” Tommy’s real name is Corey Feldman.
Kat’s Explanation: If you watch her lips she does seem to be saying Tommy even though it sounds a little like Corey.
Mrs. Voorhees' headstone says she died June 13th 1979, but June 13th of that year wasn't a Friday.
If Part IV is only 2 days after Part III, which is only 2 days after Part II, then how did Rob get so much information on Jason, who supposedly killed his sister earlier that week.
When Trish jumps out of the window and lands, you can see that she sinks into the ground (on presumably a crash mat), but in the next shot, she begins to stand on firm ground.
Gordon, the dog, jumps out the window for no reason, after all, Jason is supposed to be down in the basement with Rob. (This scene actually brings up another blooper below)
Kat's Explanation: Maybe he saw the dead body.
If Rob was in the basement, and Jason was in the basement the entire time, how did the body get put on the doorstep? It wasn't there when Rob and Trish came in, and they pretty much went right to the basement.
Kat's Explanation: For a longer drawn out explanation, please visit the forum.
Tommy could never have shaved his whole head that fast, and considering his age, he probably never used a razor before and would have had multiple cuts.
When Mrs. Jarvis comes home, and the kids are missing, she isn't worried about them, just the dog.
When the paramedics are rolling Jason, his feet are not facing them, they are in fact sticking out at the end of the sheet. But when they come into the room with Axel, Jason has switched positions. Even if it wasn't switched around, his feet aren't sticking out anymore, and the area where Axel puts his food and papers is too high up to be Jason head or feet.
The cold box that Jason is kept in, at the hospital won't shut, yet we can see that it hasn't opened anymore, but Jason still got out. Not only that but it squeaks when opened.
When Paul dances with Tina, she takes off his cap and puts it on, but a few shots later, Paul has it back on, then she has it on again right after Sam is killed.
When Jason crushes Doug's head in the shower, his hockey mask has two red marks near the nose. But later, when he is cut in the hand by Trish, the red marks are gone.
Doug's hair changes between shots through the shower door.
When Sara goes into the second floor bedroom to dry her hair, there is a bench in the hall. Later, when Jason chases Trish upstairs, the bench has moved across the hall.
In the end, Trish swings the machete over handed, but in the next shot she knocks Jason's mask off underhanded.
Tommy shows a mask to his mom, and his hair is all messed up but in the next shot of him, his hair is fine.
When Gordon jumps out of the window glass can be seen, but in the next shot there is no glass.
At the end Trish hit Jason with a hammers back side in the neck, Jason pulls it out and throws it at Trish and it sticks not on the claw or the head it just sticks there HAMMERS ARE FLAT ON TOP !
Trish and Tommy lock themselves in his room. A purple monster mask is hanging up and gets hit by a piece of wood from the door, however, off and on it moves when it switches to another angle. It should be continuously moving, or moving and then stop for good. Unless of course it was hit again by another piece of wood.
There was no way for Jason to cut the film in the projector and then get behind the screen without Teddy seeing him move past. Also, the screen looks like it is up against a wall/bookcase, and appears to not have any room behind it.
Explanation (sent in): Perhaps Jason didn't cut it. Maybe it was the end.
When Trish raises the machete to strike Jason, she is holding it with both hands. In the next shot, she is just holding it in her right hand. This is right before his mask comes off.
When Terri goes outside to go home, you can hear her throwing up the kickstand, yet later when Tina looks out the window the bike is still upright. It couldn't stay like that without the kickstand being down, and its very doubtful that Jason would have kicked it on.
The axe being thrown at Sara seems to be aimed at her head, but it hits her in the stomach.
When Trish and Rob go into the other house they look right at the movie screen, but don't seem to see the large cut, or Teddy's blood.
Kat's Explanation: It seems that the screen is no longer down, which would explain why they didn't see his body or the blood.
Jason crashes a lamp to the floor, it gets really dark, but in the next shot the room is pretty well lit.
When Tommy spots the machete on the ground, part of it is resting on the rug, but when he picks up the machete it isn't on the rug.
Before Tommy starts hacking away at Jason after his hand twitches, the machete is still in Jason's head. But when he picks it up, its lying on the ground.
Jason was hit with the machete over his left eye in Part III, but on the cover the ax mark is above his right eye.
Tommy isn't wearing glasses while he is playing his computer game, so it can be assume that he needs them for distance. However, in the scene were he is watching Paul and Sam he doesn't have his glasses on but he can still see what is going on.
In the same scene, how would Tommy have seen that closely into the window anyhow, the houses are pretty far apart and there are many trees between them.
The outhouse looking thing, at Higgins Haven is white. But in Part III it was a dark wood color.
Tommy is looking at articles Rob has. One is titled "Murderer strikes Cafe" then "Cafe owner and wife found slain". Assuming that is Harold and Edna the article would have been wrong. It wasn't a cafe, it was a little grocery store.
You can see the crash mat buried in the mud that Trish lands on when she jumps out the window. Then you can see she is standing up on firm ground.
During Jimbo's death scene its clear that the cleaver has a cut out so it will fit his head.
Before Tina lands on the car, you can clearly see the dent that she is supposed to make by falling, is already there. It doesn't seem to leave much more of a dent after she rolls off it.
At the very end when Jason falls down on the machete, you can see his mask flex above his right eye.
Kat's Explanation: I believe the equipment was supposed to do that.
When Terri leaves you can see someone peering out the upstairs window. Jason was outside getting ready to kill her, Ted, Doug, and Sarah were downstairs, and Jim and Tina were in bed, and still in bed a few scenes later. Who would have been watching from the window?
Jason leaps in the window and grabs Tina. Assuming he's on a ledge - which there doesn't appear to be on - he wouldn't have had enough room to make a "leap" like that.
Tommy is wearing Jeans at the end, but in the next scene he's wearing shorts.
A bunch of teenagers are on vacation with drugs and beer, yet they are listening to slow romantic classical music, and slow dancing?
Trish had to break a window to leave the neighbors house? Why didn't she just step over the body?
Jason is standing on an "overhang" when he pulls Tina out, how in the world did he get up there? Not only that, but there is no overhang.
In the same scene - Tommy watched Sam & Paul getting it on through a window on the right side of the house/screen, it can be assumed that they were in Paul's room. We know Jim and Tina were in Paul's room. However, when she is thrown out the window, its on the left side of the house/screen because that is where the car was parked when Terri died. There are also no ledges under either window.
If the trail leads from the Jarvis house all the way around the lake, then why didn't Alice, Ginny or Chris run around the lake looking for help?
Kat's Explanation: Maybe they didn't know there were houses around the lake.
How did the ambulances get to Higgins Haven when Chris' van(F13th Pt3) fell through the only bridge?
Kat's Explanation: I'm sure there are others ways into the area. Not only that, but they could have moved the van.
Jason kills Sam by shoving a sharp object through the boat. The boat should have lost all air, but it didn't and was still fully intact when Paul found her.
Kat's Explanation: Its possible that no air was blown into the bottom half of the boat. Or that all the air supply had different compartments, so Jason would have had to 'pop' all the compartments before it could sink.
When Sara is trying to run out of the house, she can't get the door open. Yet when Rob and Trish go to the house they have no trouble at all with the door.
Kat's Explanation: Jason could have been holding the door closed.
Why isn't Corey Feldman in the trailer at all?
If Jason was still alive, why did he allow himself to be taken to the hospital? Why didn't he get up and leave after Chris was taken away?
Kat's Explanation: Maybe he was knocked out.
The TV in the hospital that Axel is watching doesn't appears to be an older knob model that wouldn't have a remote.