We report the intraoperative finding, at a transit-time flow measurement, of competitive flow between a venous and an arterial graft in a 72-year old woman who underwent uncomplicated coronary artery bypass grafting × 3. The blood flow in the left internal mammary artery (LIMA) improved only after temporary occlusion of the saphenous vein graft (SVG) anastomosed to the first diagonal (D1), demonstrating the presence of competitive flow from the SVG-D1 anastomosis into the LIMA–left anterior descending coronary artery (LAD) system. Interestingly the two target vessels suffered from separate critical lesions. The patient's haemodynamics remained stable throughout and no further action was taken. Her recovery was uneventful and the patient was discharged home on postoperative day 6. This case raised questions about the cost benefit of grafting a diagonal target even when it appeared to be disconnected from the LAD on a coronary angiogram.