Corrected entry: In an early scene when Jill is alone and the alarm goes off the first time, the alarm company calls moments later. In a later scene, she sets off the alarm accidently upon reentering the house and this time the alarm company does not call.
Correction: Alarm monitoring companies are like all businesses - they only have a certain amount of staff working at any given time. In the first instance, an employee of the alarm company may have had no other calls, and could respond promptly. In the second instance, all the alarm responders may have been busy (on other calls, on break, in the bathroom) and unable to call right away.
Corrected entry: Throughout the movie, people keep calling the house and hanging up, yet the babysitter acts like she doesn't know whose calling. Towards the end of the movie you see the display on the phone showing there is caller ID available.
Correction: If somebody was prank calling the house, it is very likely they blocked their number, and caller ID would be of no use.
Corrected entry: When the baby sitter calls 911, the dispatcher basically blows her off. If she was calling 911 in real life that hysterical, the dispatcher would have sent out a unit to check on her.
Correction: There was a news story a short while ago about a 911 operator being fired for doing exactly that. It's real life, not a mistake. Some news stories: http://www.jems.com/products/articles/102586/
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.
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.
Corrected entry: When Jill is alone downstairs at the beginning in the main house she is wearing her jacket. When she decides to go outside she isn't. Jill wouldn't have had time to take off the jacket when she was going straight outside and it was a very windy day and night. You wouldn't take off your jacket to go outside where it is windy.
Correction: First, it cuts to her talking on the phone and her jacket is off. Secondly, perhaps she was scared and didn't think about putting on her jacket, character mistake, not a movie mistake.