Continuity mistake: After the security elves kidnap Fred and take him to the family invervention, Fred pulls most of the wrapping paper off himself, leaving a small patch of paper stuck to his shirt on his left-hand side. This patch disappears and reappears between shots throughout the rest of the scene.
Continuity mistake: In the early part of Fred Claus, Fred throws an apple at the back of Nick's head. You see all the chunks of apple all over Nick's hood and back. However when they pan over again from Fred's point of view, there is no sign of apple anywhere, not on his hood, head or back. There isn't even any on the pillow that he is sitting on or the floor around him. It just vanished.
Continuity mistake: At the point when Santa is lying in bed and Fred is saying he would fly the sleigh, Santa has a pink slip in his hand. Through the scene, the pink slip changes from unfolded to folded. Fred then goes out the room and the camera goes back to Santa and the pink slip has gone.