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 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.
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.
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).
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.
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.
The goof items below may give away important plot points.
Boom mic visible
As Graham takes Bo inside, after Houdini has died, they meet Merrill on the way into the house. As Bo mutters "Houdini's sick", the shadow of a boom mic can be seen traveling across Merrill's shirt and face.
When Graham comes to the children after Morgan had to kill their dog, Bo is sitting with her knees up to her face. In this shot her shoes are perfectly tied. Once it changes to Graham and Morgan looking down at the dead dog and Morgan leaves, Graham goes over to Bo. Her right shoe lace is untied.
At the end of the movie when Merrill is swinging the bat, the bat breaks. It shows the break being a somewhat clean break in the middle of the bat. Baseball bats do not break that way since they are made with the grain of the wood. This causes them to splinter when they break, and they break at the weakest part of the bat - the handle.
Aliens are supposedly afraid of or damaged by water, yet they can also run naked through the best way for a body to become soaked: a Pennsylvania cornfield at night. It would be highly, highly unlikely (if not unheard of) that there would be no dew on the corn at night.