In dual-energy X-ray absorptiometry, accurate identification of the soft tissue area is essential for establishing baseline attenuation and ensuring precise bone mineral density measurement. This study analyzes a 75-year-old female patient who demonstrated an abrupt decline in femoral T-scores within one year. Daily quality assurance of the device and operator precision (least significant change: 0.017 g/cm²) were within normal limits. However, due to an auto-segmentation error, the soft tissue area was lost (area = 0), and a high-density neutral zone was misclassified as the background reference, resulting in excessive subtraction of bone signal. Although the original dataset was immediately deleted due to clinical workflow demands, limiting detailed retrospective analysis, a reproduction experiment using a volunteer successfully demonstrated the same algorithmic error mechanism.