Audio problem: When Scott is making a distraction, he says, "Well, this show stinks." But his mouth is saying, "Well, this show sucks." Done in order to keep the G rating, I presume.
Continuity mistake: The second time Jack and Santa fall from the sky and are laying in the snow outside the townhouse, it cuts to the scene from the The Santa Clause where Scott Calvin first puts on the coat. However, in the original movie, Scott doesn't put the coat on until after he and Charlie have left the townhouse in the sleigh and are on the roof of the first house in which Scott goes down the chimney.
Continuity mistake: Santa is holding the snowglobe as Jack Frost enters the scene. As Santa bends down to speak with Lucy, his hands are empty and Lucy is holding the snow globe and she proceeds to give it to Santa.
Continuity mistake: About 8 minutes into the movie, there is a news rolling display in the background which is rolling from right to left which is normal for the English language. In a close-up of Mrs. Claus, she has an expression on her face and the news scroll is going left to right behind her which doesn't work for the english language.
Revealing mistake: During Jack Frost's "Santa Show," the shot showing him just as he turns his head while walking down the stairs, a piece of the mask he was wearing came loose and a distinct shadow can be seen.
Revealing mistake: When Neil is teaching Yoga exercises to the elves a first close up shot of one of the elves (the Asian looking one) shows he doesn't have pointed ears like the rest in the group.
Revealing mistake: When Lucy is going into the snow globe collection there is a spark reflected in her eye that goes over her eye lid, which shouldn't, that is a FX Problem.