Coumarins and heparins are commonly used for temporary or long-term anticoagulation. These molecules have potentially devastating side-effects, including widespread skin necrosis. We report the case of an elderly patient under oral anticoagulation with coumarins, who developed widespread necrotic cutaneous lesions upon introduction of intravenous and subcutaneous unfractionated heparin administration for a surgical procedure. Laboratory investigations revealed the presence of circulating antibodies directed against heparin-platelet factor 4. The lesions slowly resolved after withdrawal of heparin, whereas oral coumarin was re-introduced without complications. This case illustrates the rare occurrence of skin necrosis as a result of unfractionated heparin in a patient under chronic coumarin medication. Recognition of this rare complication and appropriate laboratory testing is mandatory for prompt institution of alternative anticoagulant therapies.