dead water pump
Not a good thing to happen but not worst.
I've had them crap out and water and steam everywhere. I've been told that coolant can actually burn. If that's the case problem could be grave.
Also, if it crapps out and you don't notice (for what ever reason there is not steam) your temp gage climbing, you know the obvious answer there. Engines goes from hummmmm to clunk-stop. Saw a guy seize a taurus, not a pretty thing.
His wife was driving and when I asked her did she notice the temp rise she said yes but the car was running ok so she figured it was no big deal.
I never saw coolant boil like that, scary sight, but not too scary because it was not my car.