One of my top favorites is 'Unbroken Chain' on the Grateful Dead's album 'From the Mars Hotel'. This song immediately came to mind since it was only done live a couple of times. To paraphrase Bob Weir' explanation as to why it's never performed live; 'Unbroken Chain' presents a problematic situation causing it to stand apart from our other songs. The harmonic lead comes from Phil Lesh and his melody in the lower registers. Low note frequencies travel at a considerably slower rate of speed than do frequencies in the middle and high registers. This makes the audibility and the cohesion difficult to reproduce in a live environment.