Continuity mistake: After Jimmy draws the pentagram on his hand to connect the dots, he holds up his hand to show his sister and the drawing is different from the one shown before. The outline is much darker.
Revealing mistake: When Christina Ricci picks up the silver knife off the floor in her house and her hand starts to smoke, pay attention to the shadow. You can see smoke coming from her wrist before her hand touches the knife.
Continuity mistake: When Christina is talking with the blonde girl in the office (after the first werewolf attack) the blonde's hair is down on one side and tied on the other. Right after the last shot of Christina, suddenly her tied up hair is down and flawless.
Continuity mistake: When Bo goes to kiss Jimmy after telling him that he is gay, the facial shot of him shows his head moving to kiss Jimmy by turning to his right. But in the shot when we see behind Bo his head is turned to his left.
Continuity mistake: When Zipper takes the steak off the counter, its obviously raw, but when he has it in his mouth and walks away with it, its completely stiff as if it's frozen.
Continuity mistake: When Jonie is killed as werewolf, she lands on her back. When the police officers come to look at her, she's on her side.
Answer: I assumed it was Jake based on what transpired in the previous scene and his face was the last one shown before the elevator door closed. Jake told Ellie he didn't want to lose her, they had something "special", and asked if she would bear with him until he overcame some difficulties. Jake saw Jenny as a pest as well as someone who could ruin his still-developing (blossoming) relationship with Ellie. Jenny confronted Jake about not calling Becky anymore. Jake responded that he put himself "off the market" (was no longer available because he was pursuing Ellie), to which Jenny responded that she was "bummed" (disappointed because Jake was not free to pursue her). Jenny also put her hand over Jake's shoulder, which Jake did not like because he was not interested in her and was afraid that Ellie would see (which she may have). To top it off, Jenny kept following Jake around the room when he was talking to other people. Jenny was clingy and persistent - so had to be eliminated.
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