Yttrium-90 radioembolization is a safe and efficacious treatment option for many patients with unresectable hepatocellular carcinoma. Potential candidates for radioembolization, based on clinical criteria, undergo technetium-99m labeled macroaggregated albumin imaging to determine the extent of hepatopulmonary shunting. Dose selection is based on results from shunt imaging and can exclude patients from radioembolization therapy. We present a case of miscalculated lung shunt fraction and the circumstances that led to the critical error. Copyright © 2019 by the Society of Nuclear Medicine and Molecular Imaging, Inc.