Continuity mistake: At the end when the sled full of Christmas stuff is on the mountaintop, it starts sliding off the edge. It's going fast enough that it should be long gone by the time the Grinch reaches it, but when he does it's still right near the top of the mountain.
Continuity mistake: When the dinosaur charges toward Tumok, and Tumok lies down to shove the spear into the creature as it runs upon him, the dinosaur looks several times bigger than it did in all the previous scenes.
Continuity mistake: The huge gash in the left rear tire of the Dragula Herman repairs with his scarf and piece of tape disappears the rest of the race.