It just goes to show that too much of anything is not good for you. This isn’t the first time I’d heard this, apparently drinking too much in too short space of time overwhelms the kidneys, who can’t extract it fast enough, the blood becomes too dilute and so water moves into the cells (via osmosis no doubt, I haven’t forgotten all of my biology). This causes them to swell in size, in the brain the cells have nowhere to expand into and so crush up against the skull, which causes parts of the brain to stop functioning, namely this bits that control the breathing reflex, and you die.
Drinking several litres over a relatively short period of time could be enough to cause water intoxication. Those most at risk include people taking ecstasy, as the drug increases thirst and facilitates the release of anti-diuretic hormones so more water is taken in but cannot be excreted. Also, elderly people because their kidney function may be impaired.
So just take it easy on the heavy stuff, and eat plenty of peanuts.