When the gang are playing about in the lake, Stacy is wearing the unicorn necklace that Bobby gave her. Yet in the scene after, in school Bobby presents her with the same necklace for the first time.
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.
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 Stacy & Bobby go for a walk during their first date at the lake, Stacy's lipstick changes color from red to orange as they talk.
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.
When the police unzip the body bag at the lake, Stacy's face flinches briefly.
When Bobby shows his friends the liquor flask he brought to the dance, we hear him say, "A little flask? You like that?" but his mouth says something different.