The first time Arnie climbs the tower, Gilbert's pickup truck is parked below although he has not driven up yet.
Gilbert's hair changes length from short to long & back again several times during the movie.
At the end there is a tracking shot of the house burning down. In the foreground of this shot you will see the very same trampoline (with the red trim) that Ken Carver is jumping on earlier in the movie. In the filming process there was a scene where the Carvers give the trampoline to the Grapes which was edited out making it unclear to the audience why the trampoline is at the Grapes residence.
When Gilbert was driving the car to the police station so Mama could get Arnie the car was shown to be greatly leaning to the right indicating Mama's great weight, but when she got out at the police station the car doesn't rise.
When Gilbert is lying on the ground and Becky is asking him what he wants for himself, you can see that he has perfect teeth, not the chipped bonding he displays the rest of the movie.
After they're driving back from picking Arnie up at the jailhouse, a tractor can be seen pulling a hay baler over a bare dirt field.
Gilbert never says that his father died 17 years ago. He said that his father was "hung out to dry 17 years ago", meaning he probably lost a business or a lot of money, and a few years later committed suicide, so he really is Ellen's father. Arnie is also pretty aware of his father's method of dying. If he had died 17 years before, Arnie would just have been a baby, therefore not knowing exactly what happened.
When Gilbert finds Arnie in the bath and takes him out, you can see a towel around Arnie's waist.
There are several scenes when Gilbert is driving his truck that the shift stick on the column is clearly in park.
Every time the inside view of the truck is shown while driving with the actors, the gear selector is in park.
When the mother comes out of the country courthouse a bounce card (light reflector) is visible.
When Arnie is released from jail he says 'look' and then the camera pans to the shot of the crowd, and in the left corner, you can see a crew member with a camera (1hour 7min)
When Gilbert is looking at Becky through the grocery store shelves, you can see six or seven loaves of different varieties of Mrs Baird's bread, which is a Texas (not Iowa) bakery.