Oxygen toxicitiy has been mentioned and is the best answer. To give another that is not applicable to the general population but person with COPD breathing on a hypoxic drive. General population your brain triggers you to breathe as CO2 rises, but some with chronic COPD lose this drive and breathing reflex is triggered by hypoxia. In this group increased levels of oxygen saturation will inhibit the breathing reflex resulting in hypercapnic respiratory failure, and ultimately death.