Cal. Unemployment Insurance Code § 1184

If any refund or portion thereof is erroneously made, the director shall assess that amount to the employing unit or other person to whom the refund was made, together with any interest paid thereon, but no assessment shall be made with respect to any amount of worker contributions which the employer has refunded to his or her employees. The amount of the assessment shall bear interest at the adjusted annual rate and by the method established pursuant to Section 19521 of the Revenue and Taxation Code commencing 30 days after the service of notice of the assessment, if not paid within that period, until the date of repayment. The director shall give the employing unit against whom the assessment is made a written notice of the assessment pursuant to Section 1206. The notice shall be given within three years from the date the refund was made unless the employing unit waives this limitation period or consents to its extension. Sections 1135, 1136, 1222, 1223, and 1224 shall apply to assessments made under this section. The director shall collect the amount of any assessment made under this section in the same manner that other assessments are collected.