1. Any provision of law to the contrary notwithstanding, an unexpected decision by or order of a court of competent jurisdiction or the administrative hearing commission shall only apply after the most recently ended tax period of the particular class of persons subject to such tax imposed by chapters 143 and 144 and any credit, refund or additional assessment shall be only for periods after the most recently ended tax period of such persons.

2. The provisions of this section shall apply only to final decisions by or orders of a court of competent jurisdiction or the administrative hearing commission which are rendered after October 1, 1990, and which are determined by the court or the administrative hearing commission rendering the decision, or subsequently by a lower court or the administrative hearing commission, to be unexpected. For the purposes of this section the term “unexpected” shall mean that a reasonable person would not have expected the decision or order based on prior law, previous policy or regulation of the department of revenue.