From the article: "Drinking too much water can be dangerous; it can lead to water intoxication, or hyponatremia, in which there is not enough salt in the body's fluids outside the cells. Both conditions can lead to serious health problems, and even death."
An ex of mine had this happen. He drank several gallons of water a day, and started having chest pains. Your heartbeat is regulated by an electircal impulse that requires sodium as a chemical transmitter. He had depleted his body's sodium reserves, and was on the verge of a herat attack due to his excessive consumption of water.