Continuity mistake: During the boys' time creating the snowman, we can see that the snowman is facing the front of the house; yet he completes the transformation: he's now facing towards the trees.
Continuity mistake: In the scene just before the snowman comes to life, the clock in the hallway reads midnight. But a little later, when he is showing the snowman around the house and he is about to lead him upstairs, the clock reads 8:05.