Danger Mouse (lost original pilot of animated series; 1979)
Prior to the premiere of Danger Mouse on September 28, 1981, two pilot episodes were created around 1979-1980. One of these pilots, "The Mystery of the Lost Chord", has already been released years back but the other one, "The Great Bone Idol", is lost and has yet to see a commercial release.
Over the years, the entire episode has remained lost, though screenshots have surfaced. It was meant to be released on DVD, but only the later pilot was included. The episode was redone in 1983, but it is unknown why the original never saw a modern release.
The lost pilot might have had an animation cell reused in the second pilot, due to a few inconsistencies in the animation. The following image is the only image believed to be from the first pilot, due to the main character wearing a bow-tie in a scene where he shouldn't.
Noticeably, the other pilot was also redone, suggesting that either both were lost at one stage or that they were considered too low quality.