Plot hole: Justin is a police officer. As a result, his actions at the end of the day the first time it plays through make no sense at all. His girlfriend tell him that she has shot Nicole, and that the crime will be pinned on him because she used his gun. But he knows that there will be no gunshot residue on his hands, and there will be on hers. He knows that his whereabouts were accounted for at the time of the murder, because he was at the diner in full sight of any number of people. He knows that there is not a single shred of evidence aside from the gun, which has his girlfriend's fingerprints all over it anyway. If he were to simply arrest her now, there's no way she wouldn't be the one to go down, rightly, for the crime. Instead, he decides to tamper with the corpse and shoot two bystanders, in order to protect his innocence. It makes no sense at all.
Plot hole: How did lily pollen get on Nicole's dress? Justin's girlfriend never touched Nicole; we saw the confrontation the first day, and Nicole just shot her and then must have left very quickly because nobody saw her. Even if Justin's girlfriend were covered in the stuff, it couldn't have somehow jumped onto Nicole.