Deliberate mistake: There are a number of things wrong with the roller coaster scene (besides the obvious impossibility of a crash like this actually occurring):1) The restraints use ratchets to lock, not hydraulics. A coaster of this type wouldn't even have any hydraulic lines.2) No roller coaster has restraints that lower by themselves.3) The attendants didn't check all the restraints. This is ALWAYS necessary, on ANY ride.4) On any roller coaster, the Dispatch button (or "Start" in the movie) must be held down until the train is clear of the station. This is a safety feature to make sure the attendants are clear of the moving train. If the button is released before the train is clear of the station, it will stop.5) At the bottom of the lift, the lift chain curves to follow the track into the station. This is impossible, a lift chain cannot curve horizontally, only vertically (normally at the top of the hill).
Continuity mistake: In the scene where Kevin and Wendy are talking outside of the school, Kevin's hair change positions depending on the shot. At the same time, each time the shot goes back to Kevin, not only does his hair change, but the people behind him. It switches from Julie and Perry to random people, then back again.
Revealing mistake: The roller coaster used for the Devil's Flight is really the Corkscrew at Playland in Vancouver, where all three movies were filmed. If you look during the opening credits, you can see the name "Corkscrew" up in the lights above the coaster. After this, it says Devil's Flight during the rest of the roller coaster scenes.
Continuity mistake: At the end of the scene where Ashley and Ashlyn are burning in the tan beds, one of them looks out of the side of their tan bed. This is supposed to be Ashlyn, but she's facing the wrong way, which means she would have to be in the tanning bed she didn't get into to make this shot right.
Other mistake: When Wendy and Kevin are grabbing lunch, just before Frankie Cheeks dies, the guy asks to take their orders (via the microphone) Wendy and Kevin never reply throughout the rest of the scene, and they are at the restaurant for at least another 2-3 minutes (during the whole of Frankie's death sequence) The guy working at the restaurant must be extremely patient not to ask for their orders again.