Plot hole: Throughout the film we see several characters talk about how there's no Christmas spirit and how nobody believes in Santa anymore. If, in this universe at least, Santa does exist, it's almost impossible for people to think this. If the parents deliver the presents then how do they explain the excess gifts that Santa brings? We know that Michael is on the nice list because Santa shows him towards the end, so Michael must get presents from Santa. There is no way that Walter can't believe in Santa then because Michael gets gifts from him every year.
Continuity mistake: When Buddy is lying in the roll-out bed in the living room of his father's house and asks to be tucked in, his father does so. His father comes back up and ends up having a red thread on his shirt. The shot switches to Buddy, then back to him and the thread is now on his right arm.
Continuity mistake: After baby Buddy crawls out of Santa's bag at the North Pole workshop, he is partially sitting on the edge of the bag, grasping the leg of a teddy bear. The next shot and the baby is over a foot from the bag, and not holding on to the bear's leg, which is still in the bag.