Corrected entry: In the scene where Tiffany has snuck into the house to scare Jill this would be impossible as Tiffany wouldn't have known the address. This is apparent when the cops ask Jill what address she's at and she had to go find it. Jill's dad drove her to the house to baby-sit so she herself didn't know the street name etc, thereby making it impossible for Tiffany to know where Jill was.
Xofer
21st Mar 2006
Correction: As long as she knew Jill was at Dr. Mandrakis' house, Tiffany could have easily looked up the address in the phone book, or on MapQuest, or asked someone where the house was.
Xofer