(at around 13 mins) During the breakfast scene with the bacon, there is a box of Cheerios on the counter with the Check Mark on the box, which is to show it is American Heart Association approved. The Check Mark campaign began late 2009, although the movie is set in 2006.
(at around 39 mins) The movie is set in 2006, but during one of the earlier scenes before the 'Demon' attacks, the TV is seen in the background of the kitchen and the Dyson advert that was released in 2009 is clearly visible.
During the first 17 nights or so, you can see two things that never change/move. A white cup in the kitchen in front of the fruit plate and the pillow arrangement on the sofa. Clearly a lot of different night scenes were shot in the same night.
(at around 1h 12 mins) When Ali finds the scratches on the cellar door it is on a four panel door. When Dan goes down into the cellar and turns to look at Ali at the top of the stairs the door is completely different to the one seen before.
(at around 44 mins) When Ali is alone at the house babysitting Hunter, she hears a knock at the door and goes outside to investigate then the door slams behind her. The view we get from the CCTV Camera clearly has the time in the bottom right hand corner. But when Ali shows the CCTV tape back to Dan there is no time in the bottom right hand corner of the screen.
In one scene in the kitchen, from the overhead security camera you can see a cutting board with what looks like lemons and a knife on it. When it cuts to another camera and then back again the cutting board is gone, then after the next cut it returns.
(at around 59 mins) When Kristi is alone in the kitchen then all of the cabinet doors open at once, she then runs from the room and the cabinet doors are all closed, when she runs back in to turn off the burner the doors are still closed, but when she goes back in to clean up the doors are all open and the stuff that fell out of the cabinets is till on the ground in the way of the doors so there is no way they could have closed earlier.
Even though several evenings pass after the security cameras have been installed, a white mug is visible on the counter in the exact same spot each evening although the next day it is gone, and then back again in the late night security footage.
(at around 22 mins) When Martine runs from the kitchen to get Hunter, we see a cutting board with knives and some cut vegetables on the kitchen island. When she's walking around the kitchen with her smoking smudge stick, the cutting board and it's contents are gone. Then she leaves the kitchen with the smudge stick and goes to the foyer. Then the owners come home. When the camera angle switches back to where the kitchen is in the background, the cutting board is there again. As they all walk further into the house and the kitchen, the cutting board is still there in plain view.
In the first scene when the car is driving up to the house when they are bringing Hunter home for the first time, as the car pulls into the driveway, you can see Kristi sitting in the back seat on the passenger side, and you can see the handle of the baby carrier in the back seat on the driver's side. In the next shot where Daniel is taking the baby out of the car, the car seat is now clearly on the passenger side of the vehicle.
(at around 11 mins) At one point, Dan jokingly says he intends to "release the Kraken". This is a reference to the original Clash of the Titans, not the Clash of the Titans remake, and is therefore not an anachronism.
The goof items below may give away important plot points.
(at around 59 mins) As Kristi is in the kitchen reading her magazine whilst the kettle boils, she has a cup beside her. The cupboards fly open and she screams and runs from the room to return later to find the room how she left it. But you can clearly see the cup is not where she left it, or anywhere in sight for that matter.
(at around 1h 24 mins) At the end when Katie enters Hunter's room and throws Kristi against the camera, Katie's reflection in the wardrobe mirror clearly shows her in a different position from when she attacks Kristi, to after she drops out of view.
(at around 10 mins - unrated version) In the beginning when Kristi takes Hunter out of his highchair, it inexplicably falls over. After Kristi picks it back up, her legs appear to the actress as to be careful not trip over the wire that was used to pull the highchair down.