California Code, Revenue and Taxation Code - RTC § 4806

No refund shall be made under Chapter 5 of this part,  1 nor shall any action be hereafter commenced nor shall any action heretofore commenced be further prosecuted for the recovery, of any tax voluntarily paid which was levied prior to January 1, 1939, claimed to be erroneous or illegal, by reason of errors, omissions or illegalities in preparing, transmitting, computing, determining or fixing the budget or the tax rate or rates of any county, city and county, school district, municipal corporation or other public corporation, or political subdivision, in any case in which the taxes collected from such erroneous or illegal levy have been applied in the next or any succeeding fiscal year to reduce the tax levy for such year, or have been expended, appropriated, or applied for a public purpose.

For the purposes of this section, the mere payment of a tax under protest shall not be deemed to constitute such payment an involuntary payment.