When Bullock raises his son's coffin to his shoulder the lid is sliding around; and remains offset somewhat as he begins to walk away. As the point of view changes the coffin lid is perfectly aligned and no longer moving around.
William's coffin is called a casket twice; first by the minister and later by Sweringen. The difference is that a coffin tapers in shape toward the feet and is commonly-called a "toe-pincher" whereas a casket is a rectangular box.