In the early 1980s, implantation of bileaflet mechanical mitral valves (MMVs) in the anatomical plane was found to be associated with incomplete closure of the posterior leaflet, resulting in a higher incidence of valve thrombosis. This phenomenon, coined as the “lazy” leaflet, was avoided with anti-anatomical orientation. This historical observation helped standardize the implantation technique, but variability in surgical practice for leaflet orientation persists. The latter might have been increasing since rotatable cuff designs became available on newer generation models of MMVs. Indeed, this feature makes it tempting to prioritize a perfect custom prosthetic fit at the expense of proper orientation. We present the case of a young woman with rheumatic mitral valve disease in whom an On-X MMV was rotated in the anatomical plane, resulting in a limited excursion of the posterior occluder. This case illustrates that anti-anatomical implantation should still be favored with new-generation MMV models.