The trivia items below may give away important plot points.
When the narrator is sitting in the snow talking about night terrors, just before they finish the conversation two white figures are seen running up a hill over their right (viewer's left side) shoulders. These are the creatures encountered later in the movie and is one of several instances (indicated by camera interference).
When they first enter the door of the tunnel, they head down (right side), and if you look down (left side), a white figure is running down the tunnel. This is one of the creatures encountered later in the movie, and is one of several instances (indicated by camera interference).