Acquired haemophilia is a rare phenomenon and prompt diagnosis is essential for successful treatment. Early laboratory detection could minimize its potentially devastating consequences and reduce mortality but when a masking element such as anticoagulant therapy is present, delay in diagnosis is not uncommon. A prolonged activated partial thromboplastin time (APTT) may be falsely attributed to warfarin alone, particularly when it is associated with oral anticoagulant overdose. We describe two patients on treatment with warfarin who presented with a bleeding diathesis and disproportionately prolonged APTT, which led to the diagnosis of antibodies directed against factor VIII.