During the scene where McGonagall examines the cursed necklace, throughout the scene when she is infront of the desk or Window, they can be seen through her. And can later be seen through Snape's body. They are almost like ghosts. I've seen this in more the one version of the film, on dvd, blu-ray, in cinema and on the trailer, orginally thought it to be a single error but it is present on many fromats of the film.