A 73-year-old man underwent urgent coronary artery bypass grafting after an acute myocardial infarction. An angiogram had revealed multivessel disease with a circumflex artery lesion suspected as the primary culprit. On separation from cardiopulmonary bypass, transesophageal echocardiography revealed a new mobile mass in the aortic root. Cardiopulmonary bypass was reinstituted and a large thrombus emanating from the left coronary ostium was surgically removed. We hypothesize that the thrombus had originated from coronary retrograde extrusion during venous grafting. This case illustrates an unusual source of emboli during coronary artery bypass grafting and emphasizes the importance of perioperative transesophageal echocardiography for the prevention of potentially catastrophic outcomes.