A woman in her mid-90s underwent a left uncemented bipolar hemiarthroplasty for an intracapsular femoral neck fracture. Postoperative radiographs at 48h showed a disassociation of the left femoral prosthesis at the head–trunnion interface, with the bipolar head remaining in the acetabulum. There was no preceding trauma and the patient had mobilised postoperatively. The hip was revised to a monopolar head, and the patient's hip was protected postoperatively in a brace limiting flexion and external rotation. At 30 days following revision she was mobilising pain-free with a stable hip. Disassociation at the head–trunnion interface has never been reported in hip hemiarthroplasty, and is only described in relation to primary or revision total hip replacements (THR) following dislocation or trauma to the THR. This demonstrates a potential complication of modular prostheses for trauma.