This featurette on making the prosthetics for Game of Thrones is plenty interesting by itself, especially it features a great deal on how the show creates the White Walkers. But it also may have inadvertently gave us a peek at at an upcoming scene that might prove very important. If you saw the last Game of Thrones trailer, you know that Bran shares a scene with the Night's King, the leader of the White Walkers. Since Bran was standing in the scene, it's almost certainly one of his visions. But if you watch the featurertte, there's footage of the White Walkers and their wights where they appear to be attacking a weirwood — very possibly the one Bran is currently hanging out in with the Three-Eyed Raven.
Here's a shot from 0:55:
Could this be another of Bran's visions, or are the White Walkers really making a concerted assault on the young greenseer and his mentor?