Plot hole: In the early part of Fred Claus, the story teller explains that when you become a saint, you become "eternally ageless." However, when you get to the first scene with Santa Claus, he is complaining about the rest of his hair turning white. If he was eternally ageless, his hair would not still be turning white.
Plot hole: In the scene with the brothers group, the guys don't believe that Fred is Santa Claus's brother (because they don't believe in Santa). But why don't they believe in Santa? Santa leaves gifts for kids on Christmas. If they don't believe in Santa then where do they think the presents came from?