I don’t think it is a mistake to sometimes place an area rug on top of wall to wall carpet. Many homes (and especially condos) are mostly carpeted so defining separate areas can be more of a challenge.

Using a contrasting area rug can solve the problem by grouping an area visually. In the photo, although this area is in a separate room, it is open to the more formal space next to it. The addition of a deep, dark, textured rug emphasizes the point–This space is for reading and TV viewing.