The car they were driving towards the end of the movie was a completely different car than the one they showed rolling down the cliff.
The car the girls are driving near the end has Wisconsin plates but the mountains in the background are nothing like ones seen in Wisconsin. This could be written off as simply an out of state car, but then the car she gets a ride from while hitchhiking a minute later also has a Wisconsin plate.
When Alexis is running out the back door with the boy as hostage, before she gets to door you hear police shouting let the boy go, they are in front of house.
When the girls wreck the car and Alexis is hitchhiking it is daylight, but it is dark at the house were the police tell her she is dead.
At 20:45 when Fay is coming down the stairs looking for Jen you can see a camera man in the reflective mirror like object next to the stairs.