When Tad and Donna are trapped in the car, Donna rolls down the windows to try to let some air in. The phone rings, and Cujo attacks the car, shattering the passenger window. The passenger window is rolled up all the way when Cujo shatters it, when Donna rolled it down the scene before.
When Vic picks Tad up at his camp, we see Tad dip his finger in paint and begin finger painting. He then sees his dad and runs to him and gives him a hug and there is no paint on either of his hands or his dad's suit.
During the first night that Cujo has Donna and Tad trapped in the car, Cujo becomes enraged by the ringing phone and crashes through the window to attack it. When the phone stops ringing, Cujo remains in the window, and you can see the trainer's hand come up and pull the dog down from the window.
When the cop is driving up to the house, they show the mailbox door open with mail in it, but earlier at the post office, they state that there has been a halt put on mail delivery. However, the flag on the mailbox is up, indicating that that mail in the box is outgoing.
After the news breaks about the "internal bleeding" scare with Sharp Cereal, the ad agency comes under fire. However, the scare had nothing to do with the ad agency and they would not have come under direct fire in the way portrayed in the script. Yes, they may have been called in to change the ad campaign, or put a new spin on things, but they would not have been blamed for blood-scare because that was caused by the manufacturer, not the ad agency. The Vic character is given a half dozen or so lines of dialog "explaining" the problem to his wife, but this only makes the plot hole more evident.
At the beginning of the movie, Cujo runs after a rabbit and it goes into a cave of bats. Cujo sticks its head in the hole, and bats fly around. Watch the top right hand corner when one bat is flying around, just before he gets bitten, and you can see the string that is holding it up.
When Cujo initially attacks the Pinto, the car's passenger window shatters completely, but does not break.
The tempered glass in all automotive side and rear windows does not have the thin plastic layer in it that keeps it intact when struck (as windshields have), and this type of glass will shatter into tiny pieces and disintegrate on impact.
This is demonstrated when Mrs. Trenton uses the butt of the sheriff's revolver to shatter the Pinto's glass rear hatch, which is made of the same type of glass.
When Cujo jumps onto the hood of the Pinto the first time, trying to get inside to Donna and Tad, the movie cuts to an exterior shot of the car where Donna and Tad are nowhere to be seen through the windshield.