Php Fatal Error: Class 'yii' Not Found

We see this weird Fatal Error on our Production machines. The site runs smoothly 99.99% of the time and this and a couple of very weird errors can be found only in the error log. This is one.

“PHP Fatal error: Class ‘Yii’ not found”

"PHP Notice: Constant APPLICATION_ENV is already defined"

One after the other.

They are generally triggered by the same Apache worker PID. Any ideas about why this would be happening ? We are using Apache with mod_php with opcode cache. Has anyone seen this behaviour before ?