Cal. RTC § 4114

When it can be determined from an inspection of the tax records that the tax collector has erroneously computed the amount necessary to redeem a parcel of property as to which a redemption certificate has been issued, and such error has resulted in an underpayment of the amount required to redeem such property, the amount of redemption deficiency may be collected by the tax collector, if within four years after the date of the underpayment, the tax collector sends notice of or a bill for the underpayment by registered or certified mail to the assessee of the property for the year in which the underpayment was made, at his last known address. The notice shall show:

(1) That the tax collector made an unintended error in computing the amount required to redeem the property.

(2) That as a result of the error, the payment made to redeem such property was insufficient to pay the amount required to redeem as specified in Section 4102.

(3) In detail, the balance due.

(4) A statement that if payment of the amount due is not made within 30 days following the date of this notice, the amount due will be transferred to the secured roll prepared or being prepared and will be collected like other taxes on such roll.