When Graham looks out the window at the large flock of birds, the corn stalks are brown. This is in contrast to all of the other shots, were the corn is green.
The basement door is green in every shot except the very last one when Merrill is opening the door to go up and check for the "all clear" - it is a close-up of the handle and the door is painted brown.
Uncle Merrill disappears from the end of the table when Morgan is talking to the sheriff.
As Officer Paski walks back to her cruiser, her ballpoint pen shifts from being clipped on the outside of her shirt pocket to being fully inside.
When everyone is at the table not eating their dinner, the spaghetti on Bo's plate changes when the camera cuts to her.
The man on the poster in the U.S. Army recruiting office is wearing a U.S. Marine Corps uniform.
The Army recruiter's uniform ribbons are in an incorrect order.
Officer Paski's uniform features an American flag on the right arm which is backwards. Reverse flags are to be worn on the right arm while the left arm uses the standard flag direction. The stars of the flag are always supposed to be closest to the heart as a rule of thumb. On Officer Paski's right shoulder the flag is incorrect and a huge mistake.
There is no such agency as the Bucks County Police. All towns in Bucks County, and almost all of Pennsylvania for that matter, are served by either local police, regional police, or the Pennsylvania State Police, never by county police or sheriff's offices (which are restricted to court-related duties). The only county police force in Pennsylvania, the Allegheny County PD, is on the other side of the state from Doylestown, and furthermore, is restricted to providing law enforcement services only on county property.
They nail boards on the outside of inward-opening doors. Obviously this isn't going to prevent the doors from being opened, but anything is better than nothing and they're panicking anyway. Moreover, when Merrill asks Graham how they will know if boarding the windows will work, Graham replies, "Because they seem to have trouble with pantry doors". Graham is implying that if the aliens have trouble with pantry doors, boarding them can only help. Doing so also may give everyone, especially the children, a sense of hope and/or security.
Many viewers will ask why aliens who are harmed by water would invade a planet, unprotected, that is 70% water and has rain. It's likely that we don't know what their situation is. If they are desperate for resources, they may take a calculated risk. We already know their crop circle map was nowhere near water. It's also unlikely that they would know we would figure that out and use it against them. The fact they invaded all at once shows they were not planning on a prolonged engagement, but more of a blitz attack and would not have known if we would fight back or if we would even figure out something as simple as water would be an effective weapon.
The argument about aliens capable of flying millions of light years but not being able to open doors is not valid. At no point do we know they can't open them. The alien in the pantry isn't necessarily stuck because it can't open the door, it could just be waiting to make sure it's safe to leave. This is evidenced by the fact the aliens turned the knob on the basement door immediately. Graham's opinion that they have trouble with pantry doors is just that, an opinion and a guess. The alien in the pantry clearly got out because that's the alien Merrill beats with his bat.
The family television has no problems picking up a clear TV signal, even though the antenna cable isn't plugged in.
The Alien in the cornfield has feet that do not resemble the feet of the Alien who menaces the family at the end.
When Graham drops the knife in Ray Reddy's house (when the alien lunges at him) the hand that fumbles for the knife wears a skin tight, dark colored sleeve. Graham's jacket in the scene is baggy-sleeved and light colored.
When the Hess family goes into town, most of the trees in town have changed into their fall colors. Indeed, some leaves are brown, and there are dead leaves in the street. Yet, most of the trees around the farmhouse, especially those surrounding the corn field are dark green.
At approximately 1:38:24, the viewer can see the windows of the house broken, but the pieces of glass are too large to actually fit anybody. This would mean the aliens could not actually get in through those windows. This is also revealed in the overlooking shot out of the window, the pieces of glass being too large, and protruding into the center of the window, to fit the aliens also; especially considering the aliens are larger than humans.
The news anchor, when discussing the lights over Mexico, pauses long enough for Bo and Morgan to fight over his taping over her video. The voice returns once Morgan pops in the tape.
Graham asks Bo to turn up the volume on the TV, she reaches forward and the volume increases before she makes contact with the button. Also, she then holds her finger on the button but the volume does not continue to increase, instead, it remains constant.
When the police woman gets into her police car after telling Graham to take the family to town, the car engine can be heard starting up early, before she is fully in the vehicle, yet she didn't appear to reach out to the ignition.
When Graham is at Reddy's house, Reddy speeds away in his truck the camera is reflected in the window.
Crew members, one possibly holding a boom mic, are reflected in the lights of Caroline's police car when she is talking to Graham after they have seen the crop circles on TV for the first time.
When they drive into town, we are shown an aerial view of East Lancaster Avenue and the Oxford Presbyterian Church in Oxford, PA (near Maryland), when the area the movie is supposed to take place in is Newtown, PA (near Trenton, NJ).
In the start of the movie it says that they are 45 miles outside of Philadelphia. In reality they are no more than 10 miles from the Philadelphia border.
It is apparent that the aliens stalking the house are physically superior and agile to humans when they are chased and jump onto the roof of the house with ease. Yet to smash open doors when trapped or attempting an attack seems to be out of the question ......something a group of humans are well capable of.
On the very evening the Hess family decides to hide at home, board up windows, and isolate themselves from the world, they make four separate meals, like spaghetti, cheeseburgers, French Toast, with a large bowl of mashed potatoes. Plus there are lots of uneaten leftovers on the kitchen counter tops. Lots of wasted food for a family of survivalists.
The dog's names are Houdini and Isabella, but in the scene where they appear together in the cornfield, they are both male.
When Merrill yells at the children on TV at the birthday party in Brazil, he uses Spanish - "vámonos!" ("Let's go!"). But people in Brazil speak Portuguese, so the correct term would be "vamos!" However, it is a common misconception that Brazilians speak Spanish and could simply be an error on the part of the character.
While sheltering in the basement of the house belonging to Mel Gibson's character, he and Joaquin Phoenix's character are looking for a coal chute - something Gibson's character would have already known about, having lived in the house for several years.
The alien in the pantry appeared to scream like a human when its fingers were cut off. the alien would not make human sounds like screams, considering that they communicate using a clicking-based vocabulary.