When Kristen is trying to turn off the smoke detector, the button is facing towards the door to the house, but when the camera switches to her, then to the smoke detector again, the button is facing the opposite direction.
When James sneaks out of the house with his shotgun to investigate leaving Kristen at home by herself, we see that the time on the clock in the house is 5:05 am. Moments later after James is struck in the woods by one of the strangers, we return to Kristen in the house and the time on the clock is now 4:30 am.
Kristen goes to the shed to call for help on the short wave radio. She crouches under the table, placing the handset upright on the floor. In the next 2 shots, the handset is seen lying down. In the shot after that, just before she picks it up, the handset is upright.
When Kristen is hiding in the pantry, one shot shows her right up next to the door, looking through the slats, her back to the wall. Immediately after in the following shot, she is shown, having not moved, pushed into the cubbyhole at the rear of the pantry with her back against the food items. She is then shown again from the first view, away from the cubbyhole at the back, and back up next to the slats at the door.
The young proselytizers, identified in the end credits as Mormon missionaries, are clearly not Mormons. Mormon missionaries are at least 19, and wear name tags. They do not approach potential converts as potential sinners. No Mormon pamphlet is called "Christian Living."
One of the strangers startles Kristen at the back window. Kristen turns around and bumps into one of the dining table chairs, slamming the record player and scratching the album, which continuously skips over the same phrase. In previous shots, the record player was clear across the room from the dining table. However, the strangers move objects around to confuse their victims. The record player could have been moved intentionally.
Mike is supposed to be drunk, but shows no signs of being drunk at all. However, hours may have passed since the reception, meaning that Mike may have had time to take a quick nap or the alcohol has worn off. Also, the surprise of the object hitting his window could've sobered him up a bit.
At the end, after the boys hand over the leaflet, we see the car start up and the boys begin to walk away. On close inspection, the boy on the right clearly looks for a cue from someone behind the camera, and smiles.
When the Man in the Mask knocks Kristen out and drags her through the hallway, she is wearing the plaid shirt and jeans she changed into earlier in the movie. In the next scene, she's tied up in her jeans and boots (probably shirt too but its not visible). Then one of the strangers opens a window shade and when the camera goes back she and James are tied to the dining room chairs, and she is wearing the nightgown she wore in the very beginning of the movie. Only several seconds have passed between her tied up in her jeans and her tied up in her gown.
Toward the end of the film Mike's cell phone, which is next to his head, rings, but in the previous scene he dropped the cell phone when he heard a shot and when he came out of the car there was nothing in his hands. Also, when James realizes that he killed Mike, the cell phone is not on the floor.
In multiple shots near the end of the film, namely when Kristen attempts to run out the front door but her path is blocked by Pin-Up Girl, the piano is visible. However, both at the beginning of the film, and the end after the attackers have left, the piano is no longer blocking the door.
Kristen is dragged by Mike's dead body some time after he was killed. You can clearly see his index finger twitch. However, a body can twitch after death. Though the heart is not beating, the central nervous system still sends impulses up and down the body, causing limbs or the head to twitch.
The front door was blasted by a shotgun and hacked apart with an ax. When the boys come to the house and find the bodies, the main part of the door is intact. However, if you look towards the top, you can see where the axe and shotgun damage is clearly visible.