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 Garfield jumps on the bed, the sheets swap from slightly wrinkled to very wrinkled, depending on the angle focusing.
Continuity mistake: The dog door on Jon's front door suddenly doesn't exist when Garfield is kicked out of the house.
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.
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.
Answer: He's a cartoonist.