When Jon is waking up Garfield on Christmas Eve morning after the dream Garfield had, Jon's pants are blue at first. Two shots later, they are his regular brown color.
When Odie is in the barn looking for stuff to make a gift for Garfield, the paper bag in the bin is blue in one shot, then coral in the next shot.
When Doc Boy tries to play "O Christmas Tree" on the piano, the bench is visible. However, in one shot when Grandma tries playing and sits on the keys, the bench is gone. Moments later, when she dances off to her rocker, the bench reappears.
When Garfield, Jon, and Odie park in front of the farm house, their car is visible. Later on, when the family is around the piano singing "Christmas is Here" and we see one shot of the house from the outside, their car is gone. This is also seen in the rest of the outside shots of the house for the rest of the show.
When Jon and Doc Boy are trying to wake up Dad early Christmas morning, the covers on the bed are yellow. In the next shot, when Dad asks them what they want, the covers are gray. During Dad's line "It's 1:30 in the morning; no, it's not time to open presents yet", the covers are yellow again.
In the opening dream sequence, Jon invites Garfield to eat his way to the tree via a run of breakfast lasagnas. However, when he eats the second one, there is a brief moment when it seems as if someone used the wrong animation cels, as he stops eating that one for a split-second.