I supported my randonneuring club's 600K this weekend. My volunteer position was at the overnight control. We sent a rider to the hospital in an ambulance after he passed out while sitting in a chair. Hit the deck HARD. He was admitted to the hospital and given FIVE liters of IV fluid. He peed out ~1.5 during that time frame, so he was easily 3 liters down. Dehydration --> low blood pressure --> blackout. Fortunately, he's OK now and was released after ~16 hours in the hospital.

I sometimes chuckle when I read articles about hyponatremia (basically, over-hydration leading to low blood sodium concentrations.) Yeah, it happens from time to time, I suppose, but FAR more endurance athletes suffer from DEhydration than anything else.