The growth of new blood vessels into the normally clear cornea to compensate for chronic oxygen deficiency.

Many complications are related to the mechanical presence of the lens or a lack of oxygen (hypoxia) reaching the eye.

Swelling of the cornea, typically due to a lack of oxygen from overwearing lenses or a poor fit.

Contact Lens–Related Complications - StatPearls - NCBI - NIH

Infectious keratitis (inflammation of the cornea) is the most dangerous complication and can lead to permanent vision loss.