For some years now, there has been a campaign devoted to banning a widely-used but often lethal compound, dihydrogen monoxide. The town of Aliso Viejo, California, nearly did ban it in 2004. But the pro-DHMO lobby won out. A few days ago, Jennifer Strange, a mother of three in Sacramento, California, died of DHMO toxicity (and, more specifically, because she was ignorant of the possibility that DHMO could be toxic). She was not the first and will not be the last.
In fairness, DHMO isn't all bad. It certainly has its legitimate uses. Banning it would have adverse consequences. But every dire claim about it listed here is true. Unfortunately, when people learn that its everyday name is "water", they laugh off any possibility that it could be bad for you.
Innumerable people have told me that, for good health, one must drink n litres a day, where estimates of n range from 1 (fairly normal under most conditions) to 3 or 4 (which approaches what Ms Strange seems to have consumed). Perhaps because I am not a doctor, perhaps because I'm often skeptical of "alternative medicine", they are unconvinced by my arguments that (a) the body, for most healthy people, indicates pretty reliably whether one needs water or not; (b) the capacity of one's kidneys for processing fluid waste is not infinite; (c) consuming large amounts of water dilutes the blood, it doesn't enrich it.
While (b) can kill in the long term, it is (c) that holds the immediate danger of water intoxication. A low concentration of sodium causes an osmotic flow of water into cells, bloating them. An immediate symptom may be a headache because of swelling of neurons in the brain, causing pressure on them. Other kinds of cells may be affected too. Often the condition is not even diagnosed correctly. (Following the California case, the BBC had a good story on this.)
Marathon runners and other high-endurance athletes are particularly at risk, because they mistake the symptoms for dehydration and drink even more water, eventually putting their lives in danger.
Meanwhile, the "hold your wee for a Wii" contest that did Ms Strange in was doubly stupid: it is dangerous to drink too much water, and it is dangerous to "hold your wee", as the 14th century astronomer Tycho Brahe is supposed to have found out.
(Aside -- I wrote above of peoples' skepticism that water could be bad for you -- but until the middle ages, that was indeed the belief in Europe, as someone pointed out in a talk I attended a few years ago. Water carried deadly diseases; it was a poison even in small quantities. The Europeans, therefore, only drank ale. In Asia, they drank boiled water, or tea.)