When Doc Boy tries to play "Oh, 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 sneaks 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 no longer visible. This is also seen in the final shots of the show during the song "Old Fashioned Christmas".
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.