In the sledding scene when Clark uses the new super slick compound on his sled, you can see a rope pulling his sled when he is going through the trees it can be seen at the front of the sled. Slow motion and a dvd helps.
Not only can you see the string pulling the sled, notice the sled itself. At one point, the bottom is no longer smooth, but has long blade like things. You can see this when it jumps a snow bank and shows the bottom.
Guys, c'mon. Would you be able to dodge all those trees like that? It's a movie. It was suppose to be funny. Do you think someone would actually go all out with the lights and sh*t on the house?