Change blindness is the inability to perceive changes in images. It's a normal phenomenon, often brought about by the movement of the objects being observed. Don't believe me? Check out this video by Jordan Suchow and George Alvarez of Harvard University. Each object in the ring continues changing color or shape at the same uniform rate. Is that how it looks to you?
By the way, that illusion won the 2011 Illusion of the Year contest.