The night Garfield gets kicked out of his house and has to sleep on the porch, he looks into a window and sees Odie sleeping on the bed with Jon with no collar on. At one point when Odie is out on the porch with Garfield, he has the collar on. Then, when Odie runs off the porch to chase a scooter, the collar is gone again.
When Happy Chapman leaves the Telegraph Tower, Odie is wearing the electric collar. But when they enter the train station, the collar isn't around Odie's neck, but in Happy Chapman's pocket (as we see later).
Before Happy Chapman is approached by the dogs, cats, and rats he wraps the electric collar around Odie's neck, although he doesn't appear to fasten it. In the next few shots, the collar switches back and forth between Odie's neck and Chapman's hand.
The cat/dog flap changes from one painted the same color as the door to clear. Although the cat/dog flap might be a model where a plastic board can be inserted to lock the door in one scene it changes between shots with no time for anyone to change it.
When Jon throws Garfield out of the house because it is wrecked, Garfield scratches on the door and you can see there isn't a cat door anywhere on the door. Yet after Garfield's song, Odie climbs out of a pet door on the same front door.
When everyone escapes from the animal shelter, a comment is made by a dog about the fire hydrant located right outside the door. The fire hydrant is a "wet barrel" model which is used in warm weather locations. The mid-west uses "dry barrel" hydrants to prevent freeze damage.
When Garfield uses the remote to electrify Happy Chapman in the train station, he pushes somewhere on side of the remote control where there are no buttons, but the lights that signify the collar's activity.