Introduction: Intraocular lens (IOL) mislabeling is an exceptionally rare yet consequential manufacturing error. In this case, it was identified as the cause of a significant postoperative myopic shift. The diagnostic approach, integrating anterior segment optical coherence tomography (AS-OCT) and ex vivo lens analysis, proved essential in reconciling the discrepancy between predicted and observed refractive outcomes. Patient and Clinical Findings: A 57-year-old highly myopic patient with a history of retinal detachment in the right eye underwent uneventful cataract surgery with the implantation of a +6.00 diopters (D) enhanced monofocal IOL (TECNIS Eyhance) in the right eye. Postoperatively, the patient exhibited an unexpected refraction of −13.00 D. Diagnosis, Intervention, and Outcomes: Surgical records confirmed correct IOL selection and implantation. AS-OCT imaging revealed a steeper optic profile than that expected for a low-diopter lens. Optical modeling estimated the lens power to be +18.43 D. The lens was explanted, and ex vivo lensometry confirmed its true power to be +21.00 D. Implantation of a correctly labeled +6.00 D IOL restored the intended refractive target, achieving a corrected distance visual acuity of logMAR 0.0. Conclusions: Although this case involved an extreme refractive surprise, IOL mislabeling should be considered in any unexpected postoperative refractive outcome, particularly when clinical findings and surgical records do not align. Advanced imaging and optical analyses may be key to confirming the diagnosis.