An acute haemopericardium due to central venous catheter (CVC) insertion can be catastrophic if not recognised promptly. In the tight confines of the pericardial sac, the haemopericardium, coupled with the rare presentation of a rapidly enlarging pericardial thrombus, can propel a hitherto reversible event into an irreversible one. In the highly possible event of cardiac arrest, with the massive thrombus compressing the right ventricle, the chance of attaining return of spontaneous circulation (ROSC) is dim due to ineffective ventricular re-filling during cardiopulmonary resuscitation. In the presence of a thrombus, pericardiocentesis will be futile. Furthermore, the likelihood of re-occurrence of cardiac arrest is relatively high even if ROSC is attained. We report a case of rapidly-growing pericardial thrombus in a post-cardiac arrest patient with acute haemopericardium following CVC insertion. Timely recognition of enlarging pericardial clots causing pericardial tamponade, with the aid of point-of-care-ultrasonography, is crucial, and emergency pericardiotomy improves the failure-to-rescue index in such patients.