The Bonus Stage is an unfinished stage in Sonic the Hedgehog CD which was seemingly removed from the final version of the game. It is seen in the 1992 Sega Summer Catalogue.
Little is known about the Bonus Stage. The tilted angle of the level seems to indicate that the level was probably meant to be rotated like the Special Stage in the original Sonic the Hedgehog. Additionally, three slot machine reels can be seen in the corner. Panels with an "R" and "U" can also be seen, which is assumed to serve similar purposes to the "Reverse" and "Up" panels from Sonic the Hedgehog. More of the level can be seen below Sonic, possibly indicating that the player can fall down to lower areas of the stage.
Due to only one known picture and reference being revealed this stage, it stands to reason that the Bonus Stage was scrapped during the game's development. As such, none of this information can be confirmed and is merely speculation.