26 C.F.R. § 1.1312-8
The question whether there was an erroneous inclusion, exclusion, omission, allowance, disallowance, recognition, or nonrecognition is determined under the provisions of the internal revenue laws applicable with respect to the year as to which the inclusion, exclusion, omission, allowance, disallowance, recognition, or nonrecognition, as the case may be, was made. The fact that the inclusion, exclusion, omission, allowance, disallowance, recognition, or nonrecognition, as the case may be, was in pursuance of an interpretation, either judicial or administrative, accorded such provisions of the internal revenue laws at the time of such action is not necessarily determinative of this question. For example, if a later judicial decision authoritatively alters such interpretation so that such action was contrary to such provisions of the internal revenue laws as later interpreted, the inclusion, exclusion, omission, allowance, disallowance, recognition, or nonrecognition, as the case may be, is erroneous within the meaning of section 1312.
26 C.F.R. §1.1312-8