Excessive anticoagulation with warfarin may contribute to certain complications, including bleeding into body cavities. Haemopericardiac tamponade secondary to warfarin is rare outside cardiac surgery. The present report describes an unusual presentation of spontaneous cardiac tamponade in a patient on warfarin and recently treated for chest infection with erythromycin. The patient was referred to the surgeons with acute abdominal pain and hypotension. Blood tests revealed an international normalised ratio (INR) of 16.9. An emergency abdominal computed tomography (CT) scan revealed pericardial effusion. Intravenous vitamin K and prothrombin complex concentrate were administered and urgent referral to a cardiologist was made for pericardiocentesis. Monitoring INR in patients on warfarin is paramount in avoiding the potential detrimental complications of excessive anticoagulation. Clinicians should be aware of drug interactions of warfarin and risk factors associated with its prolonged half-life. Internal bleeding, including haemorrhagic cardiac tamponade, should be ruled out in patients with unexplained hypotension and excessive anticoagulation.