In Chicago, the kindly old man is standing next to a red donation kettle on a tripod. Both the kettle and the tripod are covered in snow, but the man collecting money does not have any snow anywhere on himself, not even on his shoes. Donation kettles are never left out alone and even if he took over from another attendant, he would have walked through a sidewalk of snow and picked up snow on his shoes.
While shopping at Frederick's Toy Store, the color of the alien toy gun in Stacy's left hand changes from blue to red and back to blue.
When Edward and Stacy cut the wedding cake, everyone claps. The right camera shot shows Kevin using his right arm to hug Olivia while she claps and holding Margaret's flowers in his left. The immediate left camera shot shows Kevin's hands empty and clapping vigorously, while Margaret holds the flowers in her left hand.
When Stacy, Kevin and Olivia arrive at the cottage for the first time, it is already decorated for Christmas, including a large Christmas tree.
On Margaret's first day of swapping places, she burns breakfast. Olivia suggests they go out to eat at a cafe near the square where they got the Christmas tree, but they never bought a Christmas tree, only Christmas stockings.
When Edward and Stacy ride their horses toward the top of the hill, it's snowing and both fringe ends of her scarf are blowing backwards in the wind. In the very next shot at the top of the hill, it's not snowing at all and the scarf has been readjusted with one end hanging in the front and one piece hanging in the back. The position of Stacy's scarf ends change multiple times between shots.
When Stacy (as Margaret) is asked to play the piano at the ball, there are no pedals on the grand piano.
In the wedding scene towards the end, Margaret yells out a "woo" and put's her hands to her mouth and you are able to see Vanessa Hudgen's small tattoo on her finger, one that Margaret would not have.