Are you sure you have not accidentally pressed the 2-key?

That switches to a mode the shows both the smoothed and unsmoothed cage which has the effect you are describing.

1-key - unsmoothed
2-key -smoothed+unsmoothed cage
3-key - smoothed
4-key - wireframe
5-key - shaded
6-key - shaded+texture
7-key - shaded+texture+lights