Surface vein clot. Superficial thrombophlebitis is a blood clot in a superficial vein, just below the surface of the skin. It is most common in the leg in a preexisting varicose vein. While it can be quite painful, the chance of fatality is very small. This would oily happen if the clot extends into a deeper vein and breaks off, traveling to the lung. While theoretically possible, this would be very rare.