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) The attendants didn't check all the restraints. This is ALWAYS necessary, on ANY ride. 3) 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. 4) 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).
Deliberate mistake: During Wendy's roller-coaster premonition, you'll see a shot of Ian and Erin. The two girls behind Ian and Erin are supposed to be Julie and Perry, but are obviously stand-ins, considering they look nothing like the two actresses.
Chosen answer: They would've died, but Death "knew" they wouldn't have stayed in the car. If they had stayed, Death would change his death.