A 74-year-old woman with diabetes and rheumatoid arthritis developed a rare dual infection in her maxillary sinus (a cavity above the teeth) caused by both a fungus (Aspergillus) and bacteria (Actinomyces). Imaging tests initially suggested only fungal infection, but surgery and tissue examination revealed both organisms were present. The patient was successfully treated with surgical removal of the infected material combined with antibiotics, leading to complete recovery.