(at around 42 mins) When Shack is talking to his teammate about the Everclear, you can see several people dancing at the rave in the background. But when Jason starts chasing him, they appear to be very far away in the cornfield.
(at around 1h 22 mins) Jason is skewered on the spikes of re-bar, and yet when he is hit several times from behind with the swinging vat, the ones coming out of his chest bend all over the place. In later shots, they are shown to be quite solid and sticking out straight.
(at around 1h 16 mins) When Jason throws Linderman to the wall he gets impaled by a sharp object stuck to the wall. However, as Linderman is flying against the wall before hitting it the object isn't there.
(at around 1h 17 mins) When Jason throws Freddy against the wall, you can see that there is a large space between the window of the cabin and the end of the wall on the right. In the next shot where Jason starts to put Freddy through the window, you can clearly see that there is hardly any space between the window and the end of the wall.
(at around 1h 23 mins) When Freddy and Jason get hit by the cart, they go flying through the air and land right next to each other on the dock. In the next shot, Jason is shown getting up quite a distance away from Freddy.
(at around 8 mins) When the girls are at Lori's, just after Kia asks them if they think she needs a nose job, the lights flicker. When they flicker Kia is holding a compact in her left hand and she begins to close it. Both hands are up by her face but then the angle changes to a wider shot and her left hand is empty and on her lap and the compact is in her right hand as she places it on the sofa.
(at around 25 mins) When Lori, Kia and Gibb meet and they hug each other, we see Kia putting her arm on Gibb's shoulder. When it cuts, Lori's hand is on Gibb's shoulder and Kia's hand is on Gibb's back.
(at around 27 mins) When Lori explains she haven't had much sleep lately, Kia says "Lori, you had to sleep at the police station.". If you look at Kia's purse straps you can see some of Kia's dress sticking up between the straps. When it cuts, the piece of her dress no longer sticks up between the straps.
(at around 28 mins) When Mark is telling about Freddy to Lori we see her holding her ears, then, as Will appears, she takes her hands off her ears and lower her hands. When it cuts back to her she is still holding her ears, then she lower her hands again.
(at around 59 mins) At the institution when they find the coma patients, Will picks up some papers from the patient's charts. The way Will holds the papers differs in the shot when Lori says "Who would do this to people?"
(at around 28 mins) At school when Lori approaches Will, she puts her hand on his chin. Will then places his hand on Lori's hand and says "I thought you might be in trouble," while their hands still are on his chin. When it cuts, their hands are suddenly down.
(at around 1h 27 mins) In the end of the movie when Jason comes through the water stabbing Freddy with his own glove, you can see the spikes only coming out of Jason's chest. When it cuts to Freddy falling to his knees, the whole fist and parts of his arm-wrist suddenly has gone through his chest.
(at around 36 mins) Out at the party when Kia comes to Will and Lori and says, "Let's go shake our ass on the dance floor," you see Linderman standing a few feet away from Will. When it cuts, Linderman is suddenly close to Will handing him a beer.
(at around 1h 1 min) When Jason slams his machete in to the panel and gets electrocuted, the panel is pretty damaged. When it cuts to him grabbing onto Stubbs, it shows the panel again, much less damaged.
(at around 58 mins) In one scene a security guard gets squashed by a door. It then pans over the door, and you can see large gray marks in the door, from where it's been bashed in by Jason. The dents are quite different from when the camera pans over the door, to the wide shot.
(at around 1h 3 mins) When the teen tranquilizes Jason and gets cut in half, we see the two parts of the body in the left side of the hallway. After a few scenes, when the camera gets a close-up of the body parts, we see one in the left side of the hallway and the other one in the right side.
(at around 3 mins) In the opening sequence, when Heather (Skinny-Dipping Girl) runs down the pier, the first overhead lamp sways violently back and forth. However, in the next shot a second later the lamp is dead still.
(at around 1h 26 mins) During the final fight scene, on the dock, between Freddy and Jason, Jason pulls off Freddy's left arm. But when Lori and Will blow up the dock, the bodies of both men that go flying through the air have two arms.
(at around 11 mins) When Kia is sitting on the couch with Lori and Blake in the beginning of the movie, Blake asks Lori, "So, Lori. Am I going to get that grand tour, or what?" Kia is eating when Lori is telling Blake to get the beers. But in the next shot, when Blake gets off the couch, Kia is smiling at Blake, and then goes back to eating.
(at around 1h 21 mins) When Jason and Freddy are at Crystal Lake, Freddy begins to make torpedoes fly at Jason. When he gets hit, he is right in front of the tank, and beside the ramp. But when he looks up at Freddy, he is about two feet away from the ramp.
(at around 1h 24 mins) When Jason is being attacked by Freddy on the dock, Jason is facing Freddy in one shot. But in the next shot, he is bending over, and facing away from Freddy. Freddy then hits Jason with the machete on the back of his legs.
(at around 1h 22 mins) When the vat is swinging in the area with the ramp and cart, just before it knocks Freddy off it's moving to the right in front of the ramp. In the next shot, it's just about to knock Freddy off.
(at around 1h 16 mins) In the burning cabin, Jason hits a table and his blade gets stuck. But if you look more closely when he hit it, the blade never actually goes into the table but instead bounces off.
(at around 42 mins) In the corn field scene where Jason first catches fire after breaking one of the kid's neck. He chases Billy Bob on fire but his blade is clearly not on fire. Every time after that the blade is shown on fire.
(at around 1h 25 mins) During the last fight between Jason and Freddy on the pier, Freddy obviously pierces Jason's eyes with his claws. But during the next, close up scenes on Jason you can see his eyes intact.
(at around 16 mins) When Lori wakes up in the police station and follows the dropping blood, she turns a corner because she hears a little crying. Right after she turns the corner, the cameras show office windows. You can see someone sitting down and a camera on his knees.
(at around 19 mins) When the kid wakes up and his father is sitting beside him with his neck chopped on the porch of his house, the kid hears a noise in the bushes. As he walks towards them, we get a shot of the bushes moving - to the right of the bushes, the shadow of someone shaking them is visible.
It has been established through previous films in both series that Springwood is a town in Ohio, and Camp Crystal Lake is somewhere in or around Wessex County, New Jersey. The van sequence, therefore, would take up to at least 7 hours if we were to say generously that Springwood was on the Eastern-most side of Ohio and Camp Crystal Lake was on the Southwestern-most side of New Jersey. Even then it's already been a long night, between the massacre in the corn field and the break-in at Weston Hills. Unless Will was driving the van at a constant rate of 100 mph through the hilly roads of Pennsylvania, it seems unlikely they would reach Camp Crystal Lake before dawn, much less still in the middle of the night.
In Jason's nightmare, the campers push a young Jason into the lake to drown. Lori rushes to save him, calling for him too grab her hand. When he surfaces to grab it, she recoils in horror and screams "Jason?!" How does she know what he looks like as a child?
(at around 57 mins) When they are searching for Hypnocil on the Internet, we see a picture of the Hypnocil bottle reading "Hypnocil, 500mg, 1000 tab". We also see Hypnocil pills lying beside the bottle. Considering the size of the pills, the bottle could never contain 1000 pills. Not even close to 100 pills.
(at around 57 mins) When the drug "Hypnocil" is researched on the Internet, an article about the drug appears. If you read the article, you can see that it has nothing to do with Hypnocil. The article is about drug testing of students who engage in extracurricular activities.
(at around 43 mins) As Shack is running towards the camera in slow-motion, you can tell that he is holding a mouthful of blood, ready to spit it when he is hit by the machete. When he does spit blood, it splatters onto the camera in the lower right-hand corner.
(at around 1h 17 mins) When Linderman dashes at Jason and impales him with an American flag, Will is dragging Lori (who is supposed to be unconscious). He's dragging Lori by her arms. But when he suddenly reaches behind his back with his right arm, her right arm stays up in the air.
Several times during the movie Freddy's sweater lifts up to reveal either normal, unburnt skin, or a tight black undershirt. For example, when Freddy is hanging upside down from the large construction vat (at around 1h 23 mins).
(at around 30 mins) When Freddy does the "Got your nose!" bit to Kia, you can see at the bottom of the screen where they cut off Freddy's glove in the computer when placing it in for the final shot (full frame only).
(at around 1h 2 mins) Right before Stubbs's body is thrown through the door at the asylum, you can see where the door has been cracked beforehand in the lower left-hand side where the glass meets the wood.
(at around 1h 2 mins) After Jason has killed Stubbs, we see all the kids in the room looking for Hypnocil. Then we see Stubbs being 'thrown' through the door. However, we can see the stunt double come running before going through the door. So he isn't actually thrown.
(at around 1h 23 mins) In the fight scene at the pier, after Freddy and Jason get thrown into the air by the carriage that smacks into them, they land on the pier in sort of slow motion. As Jason lands, you can see a kneepad.
(at around 1h 16 mins) In the cabin fight scene while Jason is trying to free his machete from the table, Kia picks up a wrench and starts hitting him. When she hits him across the face, you can see the wrench bend in her hand.
(at around 44 mins) When Will and Lori are sitting in the van, the camera shows rain starting to fall in front of the headlights. But when the camera tilts up, there is a noticeable pause before the rain starts coming down on the windshield.
(at around 1h 20 mins) When Jason sends Kia's body flying into the tree during the fight scene at Crystal Lake, you can see the dummy's arm go flying off of the dummy, in the upward direction, in the shot after the body makes contact with the tree.