From the AZ Republic:
A water heater that apparently had its pressure valve shut exploded and hurtled 135 yards Thursday morning, causing “catastrophic damage” to the Phoenix home, authorities said.A report on the radio quoted a neighbor saying the garage door landed on her roof some 200 feet away.
Mythbusters tested this with spectacular results.
The water here in the desert is extremely hard and tends to foul valves with deposits, so frequent tests are a real good idea(tm). I just tested mine but the previous test was on installation about 10 years ago. There is a link in the story to simple directions on how to test your valve.