When Kevin & Laurel are eating dinner in front of the fire at the cabin, Kevin is munching a carrot stick. The carrot stick changes from thick to thin and back again as the scene progresses.
When Billy finds his old friend outside of the hotel, he has a gray sweater on. Then when they go into the hotel room, his sweater is green.
Laurel's hair changes from scraggly to almost neat when she and Kevin go inside her dorm near the beginning of the film, after she is jogging.
Laurel's weight fluctuates near the beginning of the film. This is especially noticeable when she argues with Kevin in her dorm room. In the next scene, where she is in the exercise class, she is noticeably thinner.
When Laurel is driving away from the cabin after fleeing from Kevin, we hear her car's tires screech as if they were on asphalt. But the road is dirt and gravel, and would not have made those sounds.
When Kevin pins Laurel down on the bed at her mother's cabin, she screams "What?!" at him, but her mouth never moves.