Plot hole: As we have seen in part 1, these inbreds have high tolerance to pain. They are stabbed with different objects, hit by cars, etc. Yet in this movie the mother is in labor and acting like a normal woman giving birth. If they can't feel pain, why is she carrying on like a normal woman?
Continuity mistake: When Crystal Lowe is at the lake where they are shooting the nude scene, notice that when she takes off her top and turns around towards the camera, the chain of the necklace that she is wearing is under her hands, but in the next scene the chain is in between her breasts. And afterwards when she covers herself up with her hands, the chain is back on her breast.