(at around 1h 1 min) When Julius jumps out of the ocean into the boat, they lay him down, then turn him over and move him to the bench. In the closeup shot, McCulloch's hand is on Julius, but in the next shot, both hands are on the oars.
(at around 1h 28 mins) When Jason grabs the New York cook he is several paces to the right of the mirror, but when he throws the cook into the mirror Jason can clearly be seen in the reflection standing right across from it.
(at around 1h 14 mins) When the cop gets pulled from his car by Jason, they are in an alley right next to the car. But in the next shot, Jason is standing a good 20 feet away right in front of the car.
(at around 29 mins) We are shown Jason's perspective as he is walking down a hallway towards the sauna. When Jason enters the sauna, we can clearly see there is no hallway across from the door which makes the previous scenes impossible.
(at around 27 mins) After Rennie sees Jason's child ghost the first time, she leaves her room and the camera pans down the hall to reveal Jason emerging from behind a door with a dartboard on it. Before Jason is shown, look into the pictures beyond the door and you'll see a crew member signaling for Jason to make his appearance. After Jason leaves the room you will see the crew members arm closing the door in the same pictures glass.
Charles says that Tamara had better have her Biology final done. If they're on a graduation cruise ship, the school year must already be over and thus finals would have already happened long before the cruise would have taken place.
In Friday the 13th Part VII: The New Blood, Jason's suit was shredded so much that you could see his spine and kneecaps. In this movie, he is wearing a new suit even though he was beneath the water for quite some time. He is also now wearing black gloves which he did not previously have.
(at around 16 mins) When the ship takes off we see some scenes with lots of people on board having fun playing, dancing, etc. But as soon as the killing starts the extras magically disappear and we are left with the main characters. At least after the fire alert starts the other passengers should be alarmed and rushing for the lifeboats. Surely Jason didn't just kill everybody off.
(at around 8 mins) The hockey mask Jason takes from Jim looks almost precisely the same as the mask he had previously (which was destroyed in Friday the 13th Part VII: The New Blood). It even has the same axe-gouge in the forehead.
Rennie's flashbacks suggest she was attacked by a young Jason under the lake. Jason drowned in 1957, and was a grown man by the time he began killing in the events of Friday the 13th Part 2 in 1984. Several of the films take place years apart, so this film may possibly be set in the 1990s or even later. So Rennie could either not have seen a young Jason, or she would be much older than a high school student now.
When Jim is telling Suzi the story of Jason Voorhees, he says that he drowned in Camp Crystal Lake "30 years ago". As mentioned in the previous movies, Jason drowned in 1957. Since several of the films take place years apart, this one must take place much later than the 1980s, making it a lot more than 30 years since Jason drowned.
(at around 7 mins) Throughout the film, Jason's skin is grey, slimy and decayed, except for the pinky finger on his left hand, which is completely normal, protruding through his torn glove. This is especially noticeable in the extreme close up of Jason grabbing the railing of Jim's boat near the beginning.
(at around 1h 22 mins) When Shaun and Rennie run through the two glass doors leading down into the New York subway system, you can see Jason's shadow on the wall as he stands off camera and waits for his cue to walk on screen and kick his way through the glass doors.
(at around 6 mins) When Jason is resurrected on the bottom of the lake, the water is well quite lit and everything is clearly visible, even though it's night and there aren't any other real light sources out there.
(at around 1h 23 mins) Rennie and Sean sit on the subway to get away from Jason. When Rennie sees Jason in the car ahead of them, they run toward the back. Several shots show the tunnel lights moving in the wrong direction (towards the back, which would indicate that they are running to the FRONT of the train).
In several shots, it is apparent that Kane Hodder is wearing gauntlets on his forearms to simulate Jason's decayed flesh. The skin visible near his elbow area does not match the rotted skin tone or texture of his lower arms.
(at around 30 mins) When Jason kills the boxer in the sauna, he rams the hot coal into the boxer's stomach, just above his navel. But in the overhead shot, the damage appears in the middle of the boxer's chest.
(at around 51 mins) When Wayne is being electrocuted, the sparks and fire illuminate Jason's mask, revealing that the eye holes of the mask are covered with a black cloth or mesh to prevent Jason's eyes from being seen.
(at around 51 mins) When Jason throws Wayne into the electrical switchboard, Wayne's jeans light on fire before he hits the board. Also, as the spark causes the combustion, you can clearly see flammable liquid on his jeans.
(at around 22 mins) When J.J. is killed, she drops the guitar, and the boombox suddenly stops playing too. Additionally, blood flies at a piece of glass in front of the camera before Jason actually makes contact with her.