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.
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.
Other mistake: During the premonition when the coaster is stuck on the loop, Wendy looks down to see a curved track piece with several bodies on the ground. She and Ryan then roll the coaster backwards, which results in Ryan's death and the coaster derailing. As Wendy falls, it shows the exact same piece of track and bodies despite the fact that the coaster travelled backwards a fair distance.
Chosen answer: It's metioned in the filmakers' commentary that in every Final Destination movie everyone has a premonition (Of course), well it's also mentioned by the director, James Wong, that if death wants to kill them why give them the premonition in the first place? Well he believes that there is some other force out there trying to prevent death from killing these teenagers. That's where the premonitions come in including the signs therefore, Wendy's pictures are helping signs like voices or visions from other movies. It's just the whole "giving signs" thing is portrayed differently in this movie.