This is an extract from an episode of MythBusters where they show what happens when a water heater gets over-pressurised. In the full episode they mention that it has happened to a house in America, injuring 2 people and killing a dog, where the tank went through 2 roofs and more than a hundered feet in the air.
The best way to prevent this from hapening is to check your pressure release valve every 6 months, and replace them every 5 years, like it says on the tank.
