Revealing mistake: Carl, Eddie, and Steve are out ice fishing. At one point Carl falls through the "ice" and you can clearly see that the "ice" is made of some sort of styrofoam. It looks like the stuff used for packaging. The snow looks just as fake.
Other mistake: When Eddie and Carl are out ice fishing, you can tell the background is just a wall. If you look towards the bottom, you can see the line all the way across that would make it look like a wall of scenery.