Rheumatic heart disease with moderate mitral stenosis and a large left atrial (LA) thrombus was diagnosed in a 45-year-old woman at a peripheral hospital. Oral anticoagulation was administered, and the patient was referred for further management. During the intervening period, sudden worsening of dyspnea developed in the patient and a repeat echocardiogram revealed a large thrombus freely vacillating within the left atrium. Urgent open mitral valvotomy and LA thrombus removal were planned. However, as the patient was being prepared for surgery, she had a fatal cardiorespiratory arrest and could not be revived. Warfarin is known to influence thrombus lysis, and it is possible that it may have severed the LA thrombus from the posterior LA wall by partially lysing it, leading to the formation of a ball-valve thrombus, which is a well-known risk factor for sudden death.