Dual-energy X-ray absorptiometry (DXA) is the standard method for diagnosing osteoporosis and monitoring treatment response. In addition to measuring bone mineral density (BMD), DXA images may reveal incidental osseous or soft-tissue abnormalities. Such lesions can interfere with automatic region-of-interest (ROI) placement and affect BMD measurements. We report a case in which a periacetabular calcific lesion was incidentally detected on follow-up hip DXA, and retrospective image review revealed both the onset of the lesion and an automatic ROI placement error. A 58-year-old woman with a history of breast cancer underwent annual hip DXA examinations from 2023 to 2026. During the 2026 follow-up examination, a calcific lesion was incidentally identified at the superolateral aspect of the left acetabulum. Retrospective review of prior DXA images demonstrated no lesion in 2023, initial appearance in 2024, and inclusion of the lesion within the automatically generated ROI in the 2025 examination. Therefore, the lesion was excluded from the ROI by manual correction during analysis of the 2026 examination. Orthopedic evaluation determined that the lesion was an extra-articular calcific lesion, and clinical observation was recommended. This case highlights the importance of reviewing the original DXA images in addition to BMD values to detect incidental lesions and verify the accuracy of automatic ROI placement, thereby contributing to accurate BMD assessment and appropriate clinical management.