A 91-year-old male with a history of chronic leukemia and dementia falls and sustains the hip fracture shown in Figure A. He undergoes a hemiarthroplasty through a posterior approach. A post-operative radiograph is shown in Figure B. Three weeks later he dislocates the hip arising from the toilet seat. A radiograph is shown in Figure C. The patient undergoes a closed reduction and is placed in a hip abduction brace. Post reduction radiograph is shown in Figure D. One month later he returns to clinic complaining of pain and inability to bear weight through the leg. A radiograph of the hip is included in Figure E. Which of the following factors has MOST likely contributed to the instability of the hip hemiarthroplasty?