The coronary subclavian steal syndrome (CSSS) generally occurs during follow up after coronary surgery. The case demonstrates an immediate peri-operative CSSS followed by myocardial infarction, notwithstanding a preoperative computed tomography scan quantifying subclavian artery calcifications as non-stenosing, and a subjective patent blood flow through the transected left internal mammary artery (LIMA). Blood flow inversion in the LIMA to anterior descending artery (LAD) bypass was detected by transit time flow measurement (TTFM). Following an elective brachiocephalic bypass a complementary, emergent subclavian bypass was performed, which restored antegrade LIMA flow, as confirmed by TTFM and angiography, but the patient suffered a peri-operative myocardial infarction. Reports about elective, concomitant subclavian and coronary surgery for sub-acute CSSS, allowing diagnostic investigations, have been published; however this case demonstrates diagnostic and treatment challenges in acute CSSS and emphasizes the role of peri-operative TTFM. < <b>Learning objective:</b> Despite a visible, pulsatile, and apparently patent antegrade flow through the left internal mammary artery, blood flow inversion through the completed bypass, creating acute myocardial ischemia, is possible. The recognition of this, potentially severe, ischemic complication during bypass surgery might not be evident without transit time flow measurement, due to poor clinical signs. The major learning objective is the fast recognition of this complication, preventing infarction and reducing operative morbidity and mortality in the future in similar settings.>.