Systemic artery-to-pulmonary artery fistula (SA-PAF) is a rare phenomenon that can resemble a filling defect on computed tomography angiography (CTA). SA-PAF can be due to congenital or acquired etiologies and can alter the hemodynamics of the pulmonary circulation, with the most serious reported complication being hemoptysis, requiring embolization. We describe a case of an unusual SA-PAF between the right inferior phrenic artery and the right lower lobe pulmonary artery that mimicked an unprovoked pulmonary embolus (PE) on standard CTA in a patient with cardiomyopathy. This SA-PAF was interpreted on CTA as PE due to the presence of a filling defect, revealing that not all filling defects are PE. SA-PAF should always be considered when the clinical context or the imaging findings are atypical, specifically with an isolated filling defect visualized in the inferior lower lobe pulmonary artery. The false-positive PE was the result of mixing of systemic non-opacified blood with opacified pulmonary arterial blood.