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 Charlie Brown holds the door for Snoopy before the gang leaves to trick or treat, his costume doesn't have all the extra holes in it like it does in the scenes before and after.