Rivaroxaban is an orally active direct factor Xa inhibitor used to treat venous thromboembolism with approved starting dose of 15 mg twice-daily. We present a case of an accidental overdose in a patient with pulmonary thromboembolism, when the patient received two 150 mg doses of rivaroxaban, instead of 15 mg as prescribed, given 12 hours apart. This error was recognised ten minutes after the second dose, when 50 gm oral activated charcoal was given. Rivaroxaban was stopped and rivaroxaban concentrations, INR, and APTT were monitored. The overdose was uncomplicated and 15 mg twice-daily rivaroxaban was restarted on day two. Apparently unlikely and potentially hazardous dispensing errors do happen. Each oral anticoagulant has a different dosing schedule. In our patient, the prescription for 15 mg twice-daily rivaroxaban was misread as 150 mg twice-daily (a correct dose for dabigatran in atrial fibrillation). Such errors are preventable. Prompt administration of activated charcoal under monitoring of a specific rivaroxaban assay can greatly help management of unusual situations like this one.