Closest I ever came was on an excessively hot day when I'd been standing in full sun for about 20 minutes at school. My vision faded and then went out (nothing unusual for me, I suffer migraine blindness)... but then my hearing began to fade and the blindness was getting worse. Felt dizzy and kinda sick, got to the point where i could only just hear myself speaking before I managed to sit down. Was fine a minute or two later, though.