Failure of manipulative reduction of acute anterior dislocation of the shoulder is extremely rare. A 55-year-old man dislocated his right shoulder when he fell heavily. Initial radiographs and computed tomographs demonstrated an anterior dislocation with fracture of the glenoid rim. Several attempts at closed reduction were unsuccessful. At the time of open reduction, the cause of failure was found to be interposition of a fragment of the anterior inferior glenoid rim in the joint. To prevent redislocation, the fragment was held in place by two Herbert mini bone screws after anatomic reduction, and the ruptured subscapularis was reattached to the lesser tuberosity. Two and a half months after surgery, the shoulder was stable with full range of motion. To the best of our knowledge, this is the first reported case of interposition of a fracture-fragment of the anterior inferior glenoid rim causing failure of reduction.