The case concerns a 66-year-old man with systemic lupus erythematosus and previous invasive pulmonary aspergillosis who presented with fever, headache, and cognitive decline. Imaging revealed a cavitary lesion in the lung and brain nodules that initially suggested a recurrence of aspergillosis. Despite intensive antifungal treatment, the patient's condition deteriorated rapidly. A PET-CT scan subsequently showed intense hypermetabolism in the lungs, brain and bones, which pointed the diagnosis towards malignancy. A lung biopsy confirmed stage IV poorly differentiated squamous cell carcinoma of the lung with brain and bone metastases. The case highlights the risk of diagnostic delays that can arise when clinical thinking is directed at prior infection. The authors emphasize the importance of recognizing warning signs such as treatment failure and using histopathological confirmation to overcome diagnostic biases in immunocompromised patients.