(at around 5 mins) When Louis is checking on Ellie after she fell off the tire swing he is wearing a tee shirt without a collar and sleeves that are rolled up midway past his elbow. When Rachel gets up to rush after Gage his tee shirt is now an open shirt with stripes and a collar. In the next shot when he gets up to follow Rachel his shirt is once again back to a tee shirt.
(at around 1 hour 2 mins) When Louis goes to the cemetery, the flowers on the grave move and change color between shots.
(at around 1h 25 mins) When Louis falls out of bed, he hits his head on a nightstand, dragging a pillow off the bed when he falls. The pillow is shown very clearly on the floor between the bed and the nightstand. A few shots later the pillow is back on the bed again.
(at around 4 mins) When the Creeds arrive to their home, Ellie runs towards the tree-swing and her hat falls off. In the next shot, she is still running and her hat is back on.
(at around 5 mins) When Louis is checking on Ellie after she fell off the tire swing his pant legs are down. When Rachel gets up to rush after Gage his pants legs are now rolled up. When he gets up to follow Rachel his pants legs are once again down.
(at around 49 mins) The fateful tanker truck displays a "Dangerous" placard. On a tanker the placard would describe the type of product being hauled (i.e. "Flammable", "Corrosive" etc). The "Dangerous" placard is reserved for mixed hazardous materials. You don't mix on a tanker.
After Victor Pascow's death, Dr. Creed throws his patient file into the trash can. No patient's file should simply be thrown away, especially after the patient was involved in an accident which would require a full investigation (including the use of his medical records).
(at around 57 mins) When Bill Baterman is looking at his son Timmy's Purple Heart, the enclosed ribbon and lapel pin in the case are for a completely different medal. The ribbon should be identical to the cloth portion of the Purple Heart.
During the funeral, Gage's body is shown to be intact and in a suit when the lid comes open. Being hit by something as large and heavy as a loaded gasoline tanker going full tilt would have caused Gage's whole body to practically explode against and around the front of the trunk upon impact due to the human body being mostly water and made of soft tissue. At absolute best, the force of the impact would have severed his legs from the knee down.
Jud's house was set on fire in the afternoon and is still burning by sundown, yet none of the truckers who drive by have called in the fire. This is impossible as Maine has a "duty to report" law, meaning that it's citizens are required by law to report a fire.
When they first arrive at their new house and Gage is next to the car's trunk you can see cat food in the bottom right corner of the screen used to get a reaction out of the cat.
In the outdoor scenes where Louis is alone on Thanksgiving weekend, the trees in the background have leaves on them. In Maine at Thanksgiving time, the trees would have been bare.
When Louis is sitting in the tub, the wall behind him shows signs of splashed water before the rat falls into the tub, likely from prior takes.
At the end of the movie, when Gage's body is lit ablaze, it's clearly a doll and not a real body.
In the picnic scene preceding Gage's death, Rachel and Ellie are both wearing sleeveless and backless dresses, and Gage is wearing short sleeves, despite this scene taking place sometime after Thanksgiving in Maine. In the book, this scene takes place the following spring, but that is never made clear in the movie.
When the security car shines its spotlight at Gage's grave, it fully illuminates it, so there is no way that anyone inside the car could miss seeing Louis standing in the grave, or the mounds of dirt piled around it.
In the German version, Gage's voice sounds like a 7-year-old child, but his laugh sounds like a baby / 1-year-old.
After Rachel tells Louis about how she had found Zelda dead, Louis refers to Zelda as "clinically insane". The word "insane" has been discarded by clinicians as stigmatising for many decades, since well before the film was made. It is very unlikely that a doctor would use this term to describe Zelda, especially when trying to comfort Zelda's sister over her death.