Warpage of the cornea. Result of wearing contact lenses improperly or for too long. This is physical warping, and can be seen with the naked eye. The cornea is, for the most part, a smooth curved surface, however, when contact lenses are overworn (causing hypoxia), a side effect includes the warping of the cornea, through corneal edema (warping of the corneal tissue). This changes the shape of the cornea, and affects vision.

If you're in a gutsy mood, go look for pictures of corneal warpage and refractive error changes. They are NOT pretty.