We present the case of a man in his 50s with a history of recurrent unprovoked venous thromboembolism who developed haemorrhagic shock due to venous intestinal ischaemia. Despite anticoagulation and monitoring of anti-Xa levels, the thrombosis progressed, leading to complete necrosis and perforation of the small bowel wall, persistent septic shock and ultimately death. The clinical course and family history raised suspicion for hereditary antithrombin deficiency, a rare but potentially catastrophic thrombophilia. This case highlights the importance of considering inherited thrombophilia in critically ill patients with progressive thrombosis despite adequate anticoagulation, and the challenges of managing these conditions in the intensive care unit.