Our water heater was doing something weird. A valve on the side, with a tube running down from it was leaking very hot water onto the floor. We turned off the water to the water heater to prevent a major catastrophe.
According to the owners manual, the valve is a safety valve that opens up if too much pressure builds up in the water heater. This happens if the water overheats and can be very dangerous. 55 gallons of almost boiling water could drain onto the basement floor.