Primary headache disorders account for the majority of the cases of headache. Nevertheless, the primary objective of a physician, when encountered with a patient with headache is to rule out a secondary cause the headache. This entails a search for specific associated red-flag symptoms or signs that may indicate a serious condition, as well as a heightened suspicion of and evaluation for a don't miss diagnosis. We present a case of a high-school student whose first manifestation of systemic lupus erythematosus (SLE) was a headache due to cerebral venous and sinus thrombosis, initially misdiagnosed as tension-headache and 'ophthalmoplegic migraine' (now known as 'recurrent painful ophthalmoplegic neuropathy'). The patient made a complete neurological and radiological recovery after systemic anticoagulation and treatment of SLE. An analysis of the clinical errors and cognitive biases leading to delayed referral to hospital is presented. We highlight the fact that adherence to the fundamental principles of clinical medicine and enhancement of cognitive awareness is required to reduce diagnostic errors.