Sediment in a hot water heater.
What happens then is water displacement. The sandy partials that collect on the bottom of the tank displaces the water ever so slightly. This results in hot spots on the bottom of the tank. When this occurs the flames overheat the tank and begin to break down the steel. After many years this breakdown ends up becoming a tiny hole that is filled with this debris, oftentimes preventing the leak.
However if you decide to drain your heater after many years of not doing it, or all of a sudden you start using the heater more then normal, this sediment that has been plugging that tiny hole is flushed out, often resulting in a leak in a few days after.
So the moral is to flush yearly, but if you have not done so for many years to not touch it or you will end up with a leak.