Foreign body ingestion can be caused by many factors, including hallucination in patients with mental retardation. Most ingested foreign bodies are naturally discharged, though surgical intervention is necessary in some cases. Endoscopic intervention often leads to successful outcome, though open surgery may be required in certain instances. A 29-year-old mentally retarded woman presented to emergency services with a 2-day history of right lumbar pain that increased with movement. Physical examination revealed no specific sign beyond palpable tenderness in the right lumbar region. Radiological examination revealed a metal nail in the upper right quadrant, stretching obliquely toward the retroperitoneum. Endoscopy failed, and the nail was extracted via laparotomy. Foreign body ingestion may occur in patients of any age, but is more common in the pediatric population and in patients with mental retardation. Commonly ingested foreign bodies include daily objects, toys, and dentures, though they may differ in patients with mental retardation. The treatment of such cases requires a diversity of methods and experience. Foreign body ingestion should be kept in mind when a patient presents with pain in the right lumbar region, particularly in patients with mental retardation.