We present a case of early venous congestion in a superior gluteal artery perforator flap for breast reconstruction. The case is unique because the early detection was based on a paradoxical handheld Doppler signal. During reexploration, the vessels were compressed with intermittent obstruction on inspiration. The vessels were repositioned and the flap was salvaged with no further complications. The pathophysiology of this paradoxical sentinel sign is discussed, along with other reasons leading to the outflow obstruction.