Neb. Rev. Stat. § 77-2787

An erroneous refund shall be considered an underpayment of tax on the date made, and an assessment of a deficiency arising out of an erroneous refund may be made at any time within two years from the making of the refund, except that the assessment may be made within five years from the making of the refund if it appears that any part of the refund was induced by fraud or the misrepresentation of a material fact.