The slip-up is near the end of the movie, where Garfield and Odie are in the train station where all the luggage is at. They get trapped by the dog napper/morning news host. when the dog napper grabs Odie he then puts the shock collar on him.

Then the camera cuts to the angry sheltered dogs that appeared from behind the luggage. Then when the camera cuts back to the dognapper, he has Odie in his hand but the shock collar is also in his hand instead of on Odie's neck.