Visible crew/equipment: In several shots during the weight room scene, a boom mic is reflected in the hanging swords.
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.
Visible crew/equipment: When both Kevin And Wendy are looking at the pictures after the funeral, Kevin asks Wendy to look at Frankies picture. Kevin then turns the car a little to the left almost hitting the tow truck, seen in the metal bumper you can see the front of the camera truck.
Other mistake: When the coaster is stuck upside down on the loop during the premonition, Wendy looks down at the track. The track shown is not what a loop would look like from above, as there is no entry/exit point; the track would not go straight across as shown.
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.