When Charlie is building the snowman, he places a "The Jack Frost Band" button onto him and we see the red scarf. In the next shot, Charlie is putting the scarf onto the snowman.
When Jack and his son Charlie are riding the toboggan (to get away from the bullies), Jack breaks the toboggan into two halves and Jack and Charlie ride separate halves down the hill. When they reach the bottom, Charlie is on a real snowboard, not half of the toboggan.
The tour sheet on the fridge shows the Denver concert as the 22nd. Next day (23rd) Charlie arrives home from school, and Jack is still traveling home. 24th is Charlie's hockey game, and that night Jack says to head to the cabin. The next day,25th, while packing, Jack gets a phone call to perform "Tomorrow", to which Gabby states is Christmas Day.
Despite the fact that they advanced ahead 1 year, everybody looks exactly the same (hairstyles, etc.)
When one boy is distracting bullies to throw snowballs at him, he has his back turned facing them. Then when he gets hit with the 'snowball slingshot', his front suddenly faces forward.
Despite the movie taking place during Winter and with snow on the ground, there is almost no air moisture shown when the characters breath.