Prior to 2012, plenty of other problems were found at San Onofre: "Problems at nuclear plant concern regulators" [utsandiego.com] in the San Diego Union Tribune covered a few of these which ended up "resulting in the simultaneous shutdown of two safety backup systems and placing operators on standby to shut down a nuclear reactor."
What was the safety problem here? From this article you referenced, the workers thought that they detected a crack in one of the safety systems and they took it offline. They overreacted and took two systems off line and prepared to shut down a safely opperating reactor because they feared the safety systems would not be adequate if there was a problem. They brought one of the systems back up in 15 minutes when they confirmed that the crack was not a significant threat. I would hardly call that a serious safety problem.