When Jill arrives at the house and Mrs. Mandrakis shows her the phone list on the refrigerator Rosa Ramirez is misspelled, it has extra letters about 10 minutes later Jill dials Rosa's number and then the correct spelling is shown.
When the Mandrakises were about to leave for dinner, Dr Mandrakis opens the door then goes back to activate the alarm system behind the door. In between shots, the door goes from open to closed magically.
When Jill is running back from the guest house, before dropping the flashlight, you see a straight, continuous beam of light in front of her however, when the camera angle changes, it reveals her flashlight bobbing up and down as she pumps her arms to run.
When Jill discovers her friend's body on the bathroom floor, the friend's left arm is draped over her torso. The camera cuts to Jill reacting, then cuts to an aerial shot of the bathroom as Jill runs out, showing the friend's body on the floor, with her left arm resting alongside her body.
After Jill sees her friends body on bathroom floor, she slides down the shower curtain sliding it to a folded state. When she gets back up to run out of the room, the shower curtain is in the exact position it was in before she pushed it aside.
After tracing the call, Officer Burroughs tells Jill the calls are coming from inside the house. When Jill drops the phone, you can see the picture of the wallpaper and 'Handset 2' on the screen with Burroughs still talking in the close shot of the phone. This would not appear if the phone was still on the line.
About mid-way through the picture, Jill calls her friend at the bonfire, but the static on the cellphone is nothing like the sounds one would expect from a real cell phone, when reception is poor. Also, the intercept she receives in subsequent attempts to call her friend, includes, initially, chimes initiated by the old Bell system decades ago for land line phones. Cell phone companies don't use those chimes, nor do they use the wording used in this movie. The recording wouldn't say: "I'm sorry, the person you're trying to reach is out of range," not since the nineties anyway.
In the scene where Jill is in the Guest House and answers the phone, when she hangs it up we get a look at the screen on it. You can clearly see that the battery is almost empty, but she just picked it up off the charging station a minute ago.