After Stacy tells Bobby it's over, she and Nicki are on the bed talking about going to a party Saturday night. The phone rings, Nicki answers it. It's Bobby and she hangs up and tosses the phone in front of her. Less than a minute later, the phone rings again, Nicki answers, but the phone is behind her
After Bobby throws Stacy into the wall, she stands up and says she is okay. Right before he gives her a kiss and says he loves her, her hair is tucked behind her ear, then as he kisses her on the forehead her hair hangs loosely.
When Stacey chases Bobby to the gym, he throws her into the wall. Later in the film, her back is shown to be covered in bruises from this. But when Bobby sneaks into Stacey's room and she is wearing a towel, no bruises on her back are visible.
Stacy's weight fluctuates noticeably in the film. When Stacy goes on her first date with Bobby, she is rather slim. In subsequent scenes in the days following, she is noticeably heavier. As the film progresses, her weight continues to fluctuate back & forth.
After Bobby kills Stacy, he goes back to the truck (where his friend is supposedly sitting) and takes out cinder blocks, tape, etc. without his friend noticing a thing. Then he goes out in a boat (evidently after dragging the body into the boat), sinks the body, and the friend only sees him as he's coming back from the lake with bloody hands.
The goof item below may give away important plot points.
When the movie opens, it shows Stacy yelling and screaming at Bobby, and we hear her scream as he kills her. This scene is shown toward the end of the film as well, but the dialog and scenes differ slightly.