Yes, it’s a medical emergency that can be very dangerous or even deadly. It can lead to seizures or a devestating condition called osmotic demyelination syndrome, which can cause a “locked in syndrome” or total body paralysis. There are certain medications that can make it more likely to happen. Severe psychiatric conditions make it more likely. It tends to happen pretty frequently to people with alcoholism too. It’s pretty rare for it to happen to people without risk factors for it because your kidneys are pretty good at keeping everything balanced even if you’re drinking a decent amount of water. Though one infamous example of that is the wee for a wii radio contest where people deliberately drank tons of water and were not allowed to pee to try and win a Nintendo wii. I believe someone died from the contest, and the organizers hadn’t realized how dangerous this was. In short, drink water when you’re thirsty, and pee when you need to pee.