It's winter and the gang goes skiing. Yet, every shot of Forman's yard shows cut, green grass & bushes. On the road, it's snowing hard enough for Eric to slide into the ditch, and the trees and the ground are covered in snow.
It's snowing and blowing as Eric and Donna are seen sitting in the Vista Cruiser. No snow is falling outside the rear window during the close-up shot.
Fez is clearly wearing some kind of cup over his groin when he is lying on the couch.
Hyde is drinking Old Milwaukee beer with the modern pull tab.
When he arrives at the valley, Fez says it's the first time he's seen snow. However, he already spent one Christmas in Wisconsin so he already saw a lot of snow.