Audio problem: When Jon says "Hot! Hot! Hot!" while taking out a microwave oatmeal, his lips don't match what he's saying. (00:02:25)
Continuity mistake: When Garfield wakes up, they keep discussing it's Monday. But when the newspaper goes to print, it says that it is Thursday. (00:03:50 - 00:20:20)
Continuity mistake: When Garfield is talking to Wally, the background has Zelda cooking Kat-Kreal Stew. In the next shot, she's serving sandwiches. (00:08:20)
Continuity mistake: When Garfield and Odie are on a fence trying to "leap" into the comic world, Odie's bone disappears in the close up shot and does not come back throughout that scene. (00:51:45)
Other mistake: Whenever the fire scene is going in slow motion, the fire in the background is not in slow motion. (01:07:00)
Deliberate mistake: In the fire scene, Jon tries to catch Garfield from the chandelier at the main staircase. When Jon reaches out to grab Garfield, there is no rail on the stairs. It comes back in the very next shot & disappears again after Garfield says "Need a lift?"
Other mistake: In the fire scene, after Garfield says "Need a lift?" You can see Wally in the corner smiling. But he's in a fire. He wouldn't have been smiling in a dangerous situation. Probably an animation screw-up.