Treatment of pseudoaneurysms by the injection of thrombin have been reported in the literature with a success rate approaching 100%. Complications have been reported with its use. We report a case where thrombin was used to thrombose a superficial temporal artery pseudoaneurysm leading to the development of seizure and ischaemia of the scalp. We advise that thrombin should be not be used to thrombose pseudoaneurysms of arteries supplying critical areas.