Revealing mistake: During the fighting scene outside the general store, Taggart hits one of the bad guys square on the nose two different times. After the second time, when the man is lying on the ground saying "Oh, my nose!", his hand's supposed to be holding his nose in pain but instead his hand is up around his forehead instead (his arm sort of blocks this but you can still see it).
Revealing mistake: It's obvious the snow is fake. You can see when Alex runs outside in his pajamas.