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. (00:00:35)
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. (00:12:55)
Continuity mistake: When the books fall from the shelf in front of Garfield, there is a pile of books right in front of him. When it cuts, the pile is gone and the books are more scattered. (00:30:40)
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 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.
Continuity mistake: When he is surfing down the stairs, he seemed to have gone further down the building than where he falls out the window.
Continuity mistake: When Garfield is looking for Telegraph Tower, he says it's next to a blue and orange tree. On the Kibbly Kat box however, there is no blue and orange tree on the box, let alone next to the tower.
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: 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.
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 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.
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.
Continuity mistake: The first time we see Jon pull his Volvo into his driveway, he parks it kinda crooked, the next time we see it, it is parked straight.
Revealing mistake: When Garfield is in the air vent, he falls out into a box, which topples over. The Garfield that falls into the box is motionless, in a crouched position, and obviously a stuffed toy.
Continuity mistake: When Odie first starts to dance at the dog show, Jon stands up, but if you look in the background while Odie is dancing, Jon is sitting down, then a couple of shots later we see him finally sit down.
Factual error: When John is talking on the phone to someone who found a dog, he says something about Odie not being a hound dog, and how sure he was. Odie is part wirehair Dachshund (you can look it up - plus I own Dachshunds), which is from the hound breed.
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.
Plot hole: If Happy Chapman is supposed to be allergic to cats how come he doesn't sneeze whenever he is around Garfield?
Answer: The song is called "Holla", by the Baha Men (of "Who let the dogs out" fame).