Sonic Boom (lost episode of animated TV series; 2014)
Sonic Boom is an animated TV show, produced by the company Sega of America, Inc. and Technicolor Animation Productions. First premiering in November 2014, it has spanned over 100 episodes in two seasons. There is, however, rumors of footage from a lost episode of this franchise.
The person who originally stated this has since deleted his original thread on 4chan, but stated the episode was a pilot shown to Sega of America, Inc., and was scrapped due to it not being "suitable for a first episode". It was never released to the public, but some have theorized that remnants of this lost episode lie in the "Meet Sticks" video.
In this video, there are several storyboard sketches that never saw a release on TV or any other form of Sonic media than this trailer. It is surprising that the sketches have never been released or even been addressed by the artists, yet it remains a mystery.