Technical error is not usually considered among the many possible causes of false-positive lung scans. A case is presented where the inadvertent injection of Tc-99m MAA through a Swan-Ganz catheter resulted in radionuclide imaging mimicking massive pulmonary embolism. After reinjection in a peripheral vein, a repeat perfusion scan showed unremarkable findings. Lack of the appropriate clinical presentation and the rarity of massive perfusion defects are important clues to recognizing this unusual but misleading situation.