Continuity mistake: After Cassie has met Shawn at the band stand, Shaun reaches out and holds her right hand, yet when it zooms in for a close shot of her, she is twiddling her scarf with her right hand, yet when it zooms out again, Shawn is holding her hand again.
Continuity mistake: Raven, Annabel's androgynous-looking lover, has a tattoo of a cross on her right shoulderblade - you see this as she is walking away from the camera into the gothic rave club. However in the scene in the library where Cassie catches Annabel and Raven in a cinch, Raven turns over a few times and the tattoo has magically jumped from the right side of her upper back to her right collarbone (on her front).
Continuity mistake: Close to the end of the movie where Cassie and Raven are lying next to each other in their hospital beds, a nurse comes in and covers up Ravens boob, then they switch over to Cassie and then back to Raven and her boobs uncovered again, then they move back to Cassie and back to Raven again and its covered up again with the sheet.
Answer: I believe it's both a red herring (Oh, another slasher teen horror) and an attempt at making it more believeable that there are these killers chasing Cassie throughout her 'dream'.