Coronary artery bypass graft (CABG) surgery is one of the main treatment procedures for revascularization. Diagnosing postoperative complications can be difficult. One rare complication is the iatrogenic connection of the graft to the venous system, causing distal ischemia due to an arteriovenous shunt. This condition may lead to heart failure and fatal arrhythmias. Anti-arrhythmic drug-resistant ventricular arrhythmias, which can occur immediately after coronary artery bypass surgery, are sometimes related to the procedure itself. We describe the invasive management of an iatrogenic anastomosis between the left internal mammary artery (LIMA) and the great cardiac vein (GCV), which led to life-threatening ventricular arrhythmias.