Exercise-associated hyponatremia is an issue for endurance athletes. If you drink too much water it can throw your electrolyte balance off which inhibits performance. Athletes have been told to hydrate to avoid heat stroke but sometimes they overdo it.
For those who won't read the article... too much liquid intake can dilute the sodium levels in the body and lead to swelling of the brain which itself can cause all kinds of issues, some of which can lead to death.