Is corneal scarring from contact lens use serious?

Depends: Scarring is rare from contact lens over wear which usually creates symptomatic surface changes for which the lens will be removed by most. The most common scar producing effect is corneal ulcer; a bacterial culture under the lens. Most of these are peripheral and after treatment the scar causes little change. But if the ulcer is central, the scar can cause great visual changes.

Corneal scarring: Corneal Scarring is rare from contact lens over wear unless you have had a previous ulcer or significant pannus (new blood vessels from a lack of oxygen to the cornea from using the contact lenses too much) that creeps onto cornea. Be sure to use high oxygen permeability contact lenses.