Cal. Gov. Code § 21223
A retired person may serve without reinstatement from retirement or loss or interruption of benefits provided under this system upon approval of the Director of Human Resources or the governing body of a contracting agency, as the case may be, under employment by any state or contracting agency in which he or she previously served while a member of this system, where by reason of actual litigation, or a proceeding before the Department of General Services or the governing body of a contracting agency, as the case may be, or where the state or contracting agency desires to perpetuate testimony in connection with any anticipated litigation involving the state or contracting agency, and adverse interests, the services of the person are or may be necessary in preparing for trial or in testifying as to matters within or based upon his or her knowledge acquired while employed. He or she may be paid a per diem and actual and necessary traveling expenses, but he or she shall not be paid at a greater rate of compensation per diem than the rate ordinarily paid other persons by state agencies or the contracting agency for similar services. However, there shall be deducted from the per diem compensation sums equal to the retirement annuity allocable to the days of actual employment under this section.
Ca. Gov. Code § 21223