When Jon is driving into his driveway after picking up Odie, you can see into the back seat of the car and see that Garfield isn't there.
The night Garfield gets kicked out of his house and has to sleep on the porch, he looks into a window and sees Odie sleeping on the bed with Jon with no collar on. At one point when Odie is out on the porch with Garfield, he has the collar on. Then, when Odie runs off the porch to chase a scooter, the collar is gone again.
When Happy Chapman is leaving the studio with Odie for the train station, the elevator he is taking is going up not down.
When Happy Chapman leaves the Telegraph Tower, Odie is wearing the electric collar. But when they enter the train station, the collar isn't around Odie's neck, but in Happy Chapman's pocket (as we see later).
In one of the scene when Jon's car is in his driveway, the license plate is not the generic I35 749, but in the next scene it changes back.
Garfield enters the train control room when the only employee leaves the room. There would always be someone in this room when the trains are in operation.
When Garfield is walking through the vents of the tower and gets rammed into the side of the vent when the people working at the tower "Perge the system", this would be very damaging to a cat's health. This is for the fun of the movie, but this would make him not able to continue his journey.
When Garfield uses the remote to electrify Happy Chapman in the train station, he pushes somewhere on side of the remote control where there are no buttons, but the lights that signify the collar's activity.
When Garfield is out smarting Luca while getting the pie, it shows Luca's chain anchored to the house. The aerial shot shows his chain anchored to the tree by sidewalk.
When the table with the bowling ball falls, the wire that pulls it can be seen.
When everyone escapes from the animal shelter, a comment is made by a dog about the fire hydrant located right outside the door. The fire hydrant is a "wet barrel" model which is used in warm weather locations. The mid-west uses "dry barrel" hydrants to prevent freeze damage.
Despite the movie being filmed in California, Jon Arbuckle's Volvo car does not have California vehicle registration plates.
Happy Chapman is allergic to cats, but never sneezes or shows any signs of his allergy on the two occasions he is in a room with Garfield.
When Liz says that they should start looking for Odie, her dress is red. When they find Garfield and Odie, and in a lot of other scenes with Liz, her dress is the same design but it's white.
When Liz says that they should start looking for Odie, her dress is red. When they find Garfield and Odie, and in a lot of other scenes with Liz, her dress is the same design but it's red.