A high-speed cable can look healthy on an oscilloscope and still produce errors in the real system. That sounds contradictory until the two tests are separated by purpose. An eye diagram shows the electrical margin around a receiver decision, while BER measures whether those decisions actually turn into correct or incorrect bits over time. As data rates rise, the difference matters more because rare jitter events, pattern-dependent distortion, connector discontinuities, crosstalk, attenuation, and receiver behavior can hide behind an eye that appears comfortable during a short acquisition.