If you notice on most all older cartoons- they eat food but it never depletes what's on their plates, they carve meat but it never depletes what they carved it from.
They use the same animation cells over and over again in cartoons. It saves on money. You can notice this type of stuff in a lot of cartoons, such as Scooby Doo and the Flinstones.
Yup, it happens all the time in older animation. Watch an episode of The Flintstones. Ever see Barney and Fred running and about every 5 seconds the exact same background passes behind them? I don't really think of it as a slip up. Either it's done to save money or it's just easier that way.
I conquer - there are many times in this movie where the same part is played over and over again - where the Grinch is slithering along the floor, trees being cleaned off - it happens a lot.