Sonic The Hedgehog CD/806
Sonic the Hedgehog CD 806 is a near final prototype of Sonic the Hedgehog CD for the Mega CD. Unlike other prototypes the disc comes complete with cover art and was released in stores as a demo CD in Japan. The demo has a limited amount of levels playable, but this can be circumvented with a level select code.
On August 31st, 2008, it was leaked by drx via torrent. So far, this torrent is nearly dead, so it was considered as unleaked, until BetaArchive user effy11 leaked it.
Some info taken from SonicRetro.