The Chernobyl accident (power excursion) was the result of a controlled emergency shut down test, that would occur (needed in the event of a no load situation).

http://en.wikipedia.org/wiki/Chernobyl_disaster#Planning_the_test_of_the_safety_device

Although the Russian reactor did not have a containment vessel and has been criticised for being an unsafe design compared to the US designed PWR, it could be argued that a containment vessel may have resulted in the much more catastrophic ' China Syndrome ' type accident that was only barely avoided in the 3-mile island accident a few years before.