This paper investigates the determinants of intra-year error in annual effective tax rate estimates, relative to the actual annual effective tax rate (“ETR Miss”) and examines whether ETR Miss contains value relevant information. I find that ETR Miss is affected by both unbiased estimation errors related to the predictability of business fundamentals and biased estimation related to varying managerial incentives within the year. Firms with higher ETR Miss exhibit less persistent pre-tax earnings and earnings components, consistent with ETR Miss containing information on earnings quality. Finally, for firms with higher ETR Miss, investors place a lower weight on accounting earnings, consistent with the market incorporating information in ETR Miss for valuation assessments.