Continuity mistake: When Odie comes out on the porch to keep Garfield company, his collar and tags are visible; even more so when he attempts to get back in. But when the shot pulls away to show Odie alone on the steps (before running off after the scooter), his collar is gone. Part of the plot hinges on Jon trying to find Odie, who wasn't wearing his tags.
Revealing mistake: When one of the men picks up Garfield from his cage to display to potential buyers at the pound, you can see the man's right hand "entangled" with the computer-generated Garfield.
Other mistake: Jon's Volvo back license plate number in the first half of the movie is T03-851. When he and Dr Liz are looking for Odie, his front license plate number is I35-749. As they are pulling up to the train station, the Volvo in front of them also has the plate I35-749.
Continuity mistake: When Garfield is captured by Animal Control, he is put next to Happy's cat. The next time we see him in the cage, he is next to a different cat.
Revealing mistake: When Garfield and the mouse are going down the street underneath cardboard boxes, you can see the wheels of remote-controlled "cars" that were used to create the illusion. It is especially obvious in some of the scenes of the mouse, under the Chinese-Food box.
Continuity mistake: In the train station, Dr Liz is wearing white heels. As they get out of the car and embrace, she is now wearing white flats.
Continuity mistake: When Garfield rescues Odie and Happy Chapman picks up Odie he puts the shock collar on him. It then cuts to the dogs howling and then cuts back to Happy Chapman and you can see that the collar is no longer around Odie's neck but in Happy Chapman's hand. It then cuts back to the dogs and then back to Happy Chapman and the collar is back on Odie. He then starts to run and the collar is off again.
Continuity mistake: Garfield has a thumb on his front paws, giving him 5 digits on the bottom of his paws. He is shown leaning on a window and all 5 are making contact with the window as a human hand would. The paw print that Jon finds and recognizes as Garfield's are normal cat prints with only marks from 4 digits. If his paw did have 5 digits as seen in the window, he would have left a 5 toed print.
Plot hole: If Happy Chapman is supposed to be allergic to cats how come he doesn't sneeze whenever he is around Garfield?
Continuity mistake: After Garfield takes treatment at an orphanage, he looks at a dog in a cage and jumps on the counter. In this shot on the counter you can see the comb, a bottle of shampoo and a jug of liquid soap. Look at these things when Garfield says 'But I've got somebody waiting', and you'll see that the comb has changed position on the counter.
Continuity mistake: When they are doing the line up at the pound, there was more than one cat playing Happys cat. When they first put Happys cat down on the floor, you see his back is all black. But when the guard picks him back up he has a white spot on his back. When Happys cat hits the red button, his back is black again. This happens a few more times throughout the movie.
Continuity mistake: When Garfield is outside of the house, the dog door shows as green. When Odie is outside it is black. Throughout the movie, the dog door continuously changes color.
Character mistake: Garfield calls two different dogs Boomer. The first is a white dog at the vet. The second is in the scene at the train station when the dogs from the pound come in Garfield calls Spanky, the brown dog, Boomer.
Visible crew/equipment: During the opening credits, when the camera pans, looking at the photos of Jon and Garfield, you can see the camera reflected in the glass of some of the pictures.
Revealing mistake: When Garfield and Jon first bring Odie home, you see Garfield talking to one of the neighborhood cats outside while Jon has gone inside with Odie. You can see a colored square on the sidewalk where the neighborhood cat sits. Throughout different shots in the scene you see the square disappear and reappear.