From Nuclearstreet.com via K Haddam

Unit 2 of the Comanche Peak nuclear plant in Texas went offline Friday after routine testing set off safety systems.

According to a report filed with the Nuclear Regulatory Commission, workers at the Luminant plant near Fort Worth were using a blocking circuit to test a slave relay that can trip the unit's main turbine and feed water pumps under certain abnormal conditions. The relay actuated unexpectedly, according to the NRC, and the reactor shut down from full power automatically after the turbine tripped. All safety systems responded as designed.

So.. failing electronic circuitry?