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.
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.
Chosen answer: Jon makes educated guesses in the comics based on Garfield's body language and surroundings, he doesn't actually hear Garfield in either medium.
Phixius ★