Factual error: Garfield enters the train control room at Los Angeles Union Station when the only employee in the room leaves the room. There would always be someone in this room when the trains are in operation.
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.
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.
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.
Answer: The song is called "Holla", by the Baha Men (of "Who let the dogs out" fame).