(at around 35 mins) Right before Wendy says she needs a minute in private, it shows a back shot of her. She's holding the crumpled paper that has Ashley's cell phone number on it. After Julie leaves, Wendy looks to her left and the paper is out of her hands, on the desk next to her.
(at around 29 mins) When Wendy says she will recharge the camera for Julie, she plugs it into the CPU of the computer and looks at the pictures. Later, when she uploads the pictures, she plugs the cable into the camera and the other end into the CPU of the computer. She had already done this earlier, though.
(at around 4 mins) Wendy uses the schools camera for the yearbook, saying that she has to turn in the pictures for a deadline. But in the scene where she is picking up her diploma (at around 24 mins) everyone already has their yearbooks. Later Julie asks to use the camera to get pictures of graduation to meet deadline.
(at around 1h 10 mins) Kevin's photo shows a shocked expression as a camera flash has gone off close to his face. However, when Wendy takes this photo (at around 4 mins), he throws his head back, making this expression after the flash has gone off, therefore after the picture was taken, resulting in a completely different picture.
(at around 22 mins) After the real crash of the roller coaster, Wendy is seen at school getting things from her locker, one of which is an umbrella. She turns to look at the memorial wall and then proceeds to throw a bunch of stuff in the trash. Her umbrella, however, has disappeared.
(at around 1h 3 mins) During the hardware store scene, just as Ian begins his "Newton's 3rd Law" speech, he is shown to finish cutting a board and place it on rollers next to Kevin. Without a break in his sentence, the camera angle changes and Kevin is seen passing Ian the same board, which he lays down a second time.
(at around 46 mins) When both Kevin and Wendy are looking at the pictures after the funeral, Kevin asks Wendy to look at Frankie's picture. Kevin then turns the car a little to the left almost hitting the tow truck. The camera truck is seen in the metal bumper.
(at around 43 mins) The "last photo of Lincoln" is shown as Wendy points out that the crack in the picture (on Lincoln's forehead) is where he was shot. John Wilkes Booth shot Lincoln from the rear, below the left ear with a small Derringer pistol. There was no exit wound. While it is still a goof, it is possible this is a deliberate error injected into the film, as it is a common misconception.
(at around 1h 5 mins) A nail gun cannot shoot without the trigger being pulled back, and the front end (where the nail shoots out) being pushed into the wood; furthermore, electric nail guns have to be within certain angles in order to shoot. It would be impossible for the gun to shoot like it did in the movie.
(at around 13 mins) Rollercoaster seat restraints do not work with hydraulic fluid. They work with pressurized air to enable fast release in case of an emergency. The moment the braces shut, the computer checks for irregularities in the locking system prior to clearance.
(at around 41 mins) When Wendy calls Kevin at the cemetery, Wendy says she is at Jason's grave. When she is seen kneeling by the grave, the grass below her shows no sign of any kind of recent burial like the movie portrayed.
Fireworks mortar tubes must be at least Schedule 40 ABS tube, not the lightweight Schedule 80 PVC tube shown. The lifting charge would immediately explode through such a thin tube.
Fireworks are never launched without maintaining a visual reference to the mortar racks (which incidentally are banned by law to be placed on rolling trailers), allowing the pyrotechnician to immediately cease launching if there was any damage or instability to those racks.
(at around 36 mins) In the tanning bed room, when the temperature controller's display changes from 77 to 78ºF, it can be seen that the fan control switch on the lower left side is set to "On". This should normally have the fan (the one built into the wall, not in the tanning beds) running all the time. However, it is then shown how it turns on due to increased temperature in the room and makes the coat hanger wobble. The switch should be set to "Auto" if this were to happen.
(at around 14 mins) When the roller coaster is climbing the hill in the premonition, some say that Frankie disappears from the roller coaster until he is seen filming Ashlyn and Ashley on the lift. However, Frankie is still there but he is only seen for a brief moment because a very dark shadow is covering him.
For the rollercoaster accident to happen, a combination of the hydraulic fluid from Lewis' restraints being forced to close, and Frankie's camera result in the fault with the restraints and the damage to the train and the tracks. As both Lewis and Frankie remove themselves from the rollercoaster before it leaves following Gwen's premonition, the accident should have been averted.
(at around 1h 26 mins) At the very end right before the camera shows Wendy, a dead body is seen in the lower right portion of the screen and right before the camera cuts away the body is seen blinking.
(at around 29 mins) When Wendy is at her house looking at the pictures on her computer, she sees Jason's picture.He's striking the same pose as the devil and we see the roller coaster going down a hill and it looks as if it was going to hit him in the head. However, when Julie takes the picture at the beginning of the movie (at around 7 mins), the roller coaster had already passed by the devil and nowhere near Jason.
(at around 3 mins) At the beginning of the movie at the fair, Wendy is standing next to the roller coaster tracks. As the roller coaster passes her on her right side, a gust of wind supposedly from the coaster blows her hair. The wind came from the left side instead of the right where the coaster passed her.
(at around 25 secs) The roller coaster used for the Devil's Flight is really the Corkscrew at Playland in Vancouver, where all three movies were filmed. During the opening credits, the name "Corkscrew" is seen up in the lights above the coaster.
In the "Choose Their Fate" alternate version of the tanning-salon scene, Ashlyn reaches out with her left hand to pull Ashley out of the tanning-bed, just before they're both electrocuted. In the close-up shot of Ashlyn's body hitting the floor, her left hand is on the floor and empty, but in the overhead shot immediately afterwards, her left hand is still clutching Ashley's.
In the "Choose Their Fate" alternate version of the tanning bed scene when the girl standing grabs the others hand and is electrocuted her face and body are burnt but when it shows her drop to the floor dead, her skin is perfectly clear.
(at around 47 mins) When Kevin's truck rear-ends Frankie's car, the overhead shot shows the engine departing the truck and flying forward over the back half of Frankie's car. In the next shot, this time from the right hand side, you see the engine once again shoot out of the truck, and fly forward over Frankie's car.
(at around 47 mins) When Kevin's car is pushed into Frankie's car by the runaway truck, you can see two shots of the engine flying out: an overtop and a sideview. In the overtop shot Frankie's car is empty, in the sideview one, he can be clearly seen. Assuming they are the same shots, this shouldn't happen. Also, assuming he is covered by the sunblocks, one can easily notice they are semi-transparent, so at least his hat should be visible, if not his head.