At around 3 min 30 sec, the shutter on the farmhouse is hanging off. Later it's undamaged, then broken again during the attack on the house when the sheriff arrives near the movie's end.
As Brandon approaches the upturned mower, the blade can be seen spinning clockwise. After he stops the blade, the damage indicates it was turning widdershins instead.
(At around 1 hour 14 min) Brandon flew through the house, creating a hole in the wall in front of his mother who was hiding under the counter. The hole clearly expanded below the countertop with dust floating toward her. The second shot after that shows the same wall, level with the countertop, with no hole and no dust whatsoever.
When Brandon sleepwalks to the window the night he tries to break into the barn basement, his bedsheet drags behind him. In the next shot when he gets ready to jump out the window, the bedsheet is gone.
In the diner, before the fight, Erika is stacking and counting the cash register money. During the ensuing fight, Erika is swinging a bat, wind is blowing, and the money is flying around. After the scene, the cops are walking through the diner and the money is piled up nicely in neat stacks on the counter like nothing happened.
Lawn mowers usually don't have fuel pumps and rely on gravity to deliver fuel to the motor. Upside-down fuel is not delivered to the motor, which quickly dies.
Light bulbs are vacuum sealed, not pressurized. When they break there is negative force on the glass shards. Gravity alone would not be enough to force a piece into Erica's eye. Also, the piece she pulls out is perfectly flat while the bulbs are cylindrical.
When the waitress gets a shard of glass in her eyeball, she pulls it out and blood gushes from it and runs down her face. This would not happen because your eyeball is filled with a clear liquid. Not blood. The glass shard was not long enough to penetrate the back of the eyeball.
For decades, lawn mowers have had a safety addition built into the handle's grip so that if the grip is released, the motor is shut off and the blade is stopped in less than a revolution.
Near the movie's end, Brandon flies his mom up to above the clouds where commercial airliners fly 25,000 to 35,000 ft., yet she is not out of breath considering how thin the atmosphere is at that altitude, and neither show any sign of freezing when the average temperature that high is around -50°F.
Incorrectly regarded as a goof. Wild wolves did leave Kansas for a while, but have since started to return.
After the father/son camping trip, there is a scene where Brandon is on the phone with Tori, and Brandon is staring out over a vista. You can clearly tell it is not Brandon as the hand is that of a fully grown man.
Noah is clearly not wearing a seat belt so when Brandon lifts his Bronco up by the rear bumper, Noah should fall down against the windshield. One might argue that he's holding himself up against the steering wheel but there's a point where he takes his hands off of it completely yet he remains perfectly still in the seat.
The lighting (not the supernatural parts) in the barn sequences is all wrong. In numerous shots you can see the building's light fittings and none of them are ever turned on. All the scene lighting is clearly from the set lights. It comes from completely different positions and has a colour and focus you wouldn't get from the basic incandescent fittings shown.
When Brandon is asked to mow the lawn, he goes to start the mower but you can clearly see the type of push mower needs a gas bar on the handle but it isn't there.
As Noah bring his car to a stop, just before seeing Brandon standing in the middle of the road, there is a close up shot of his tire. The shot is mirrored in the vertical axis (i.e., flipped left to right). This is likely to obfuscate the fact that this is a Wrangler tire, while Noah's car is a Ford Bronco.
When Kyle and Brandon are talking after fixing the door, Kyle has two pieces of candy and gives one to Brandon. In the next scene, he still has two and when he opens and puts one in his mouth, he only had the one.
When Brandon is looking at the upside down lawnmower, in shots looking down at the mower, the wheels are not moving, but when the shot is under the mower looking up at Brandon, the wheels are vibrating.
In the school counselor's office, there is a poster stating "The world is your oyster," but the art shows the Earth inside an oyster, so the picture depicts the world as the pearl, not the oyster.
When Brandon kills his father and during the phone call with his mother, it is seen that the sun is barely setting. Later, around 1:14:37 when he shows up at the windows, it is seen that the night has come and it looks as if it was late, but these scenes have less than a few minutes of difference and the daylight is not gone that fast, nor does the night show up with stars as quick as it seemed.
The police are fully aware of a disturbance at the Breyer house due to the phone calls by Tori and the deputy. Yet no investigation is made and the plane crash came after those calls.
In class, when describing wasps, Brandon says only bees are pollinators. This is not true, wasps are very important pollinators.