What problem might flat lenses cause during wear?

Flat lenses can indeed rotate on the eye which can lead to discomfort for the wearer. The curvature of the lens is designed to match the curvature of the cornea; if the lens is too flat, it may not adhere properly to the surface, allowing it to move more than intended. This movement can cause irritation, dryness, and an unstable vision, as the lens does not maintain the correct positioning over the cornea.

Conversely, if a lens is too tight, it can create pressure on the eye and may restrict movement, leading to its own set of problems, but that does not directly relate to the flatness of the lens. In the case of stability, while some lens rotation can be uncomfortable, the proper fit of a lens should provide both stability and comfort without excessive movement. Breathability can vary with lens material rather than geometry; flat lenses could be made from materials that are either permeable or less so, but that is unrelated to their shape.