Introduction <p>Compared with fusion for degenerative disc disease, cervical disc replacement (CDR) preserves motion. We report a rare case of posterior migration of a mobile core in an unconstrained device, underscoring the risk of severe complications after disc arthroplasty. </p> Case presentation <p>A 29-year-old woman underwent Mobi-C arthroplasty at C5/6 in 2016 for refractory radiculopathy caused by a disc herniation. After initial improvement, she developed recurrent neck and arm pain at age 38, accompanied by reduced neck flexion. Imaging revealed cervical kyphosis, displacement of the upper implant endplate, and posterior migration of the polyethylene core into the spinal canal. Revision surgery showed a loose upper endplate with extensive metallosis alongside a worn, displaced core. The device was removed, thorough debridement was performed, and the segment was fused. Tissue cultures and implant sonication grew Cutibacterium acnes. She was treated with a two-week course of intravenous penicillin followed by six weeks of oral penicillin, demonstrating a satisfactory recovery at the one-year follow-up.</p> Conclusion <p>This case highlights the complexities of CDR surgery and emphasizes the need for further research into device failure mechanisms. Although the disintegration of CDR devices is rare, the potential for devastating complications warrants careful consideration.</p>