A 68-year-old man was referred for pulmonary vein (PV) isolation. Access to the left atrium was obtained via a single transseptal puncture. A heparin bolus was given immediately after puncture. Angiography of the left PV was performed. After the contrast wash-out, a contrasted longitudinal thrombus was shown revolving within the left atrium for several seconds before being expelled through the mitral and aortic valves. It is common practice in these procedures to administer an intravenous bolus of heparin immediately after the transseptal puncture. However, heparin has latency to become effective, and this can result in thromboembolic events.