For Sonic cd the iOS version .
a new level they was working on that ended up being cut for time and authenticity reasons. This image can be accessed by entering the Sound Test code 8 & 32, after which the player is taken to the level select screen as Tails.
If the PAR code FFFF08:02 is used in Sonic & Knuckles, Tails will be usable, but just about every sprite of his except the transformation sprite will be missing, basically rendering him invisible (unlike when a savestate is used; a different, glitchy sprite is used in place of Tails' original one). There is one exception: Tails' sprites used at the end of Sky Sanctuary Zone to run around the collapsing pillar are actually stored in the S&K ROM, and therefore are visible. The same sprites are also used in the same context in Lava Reef Zone.
There's a hidden area in Hidden Palace Zone that is only accessible by debug mode or having Tails in Sonic 3 & Knuckles.
Just before entering the inside of Hidden Palace, fly from the two steps left until a ledge is reached. Walk left and six power-up monitors will appear, mostly shield monitors (Ring, Bubble, Fireball). In some of the S&K prototypes, an "S" monitor is present instead of the Speed Shoes.
If you run right, leap off the ledge, and keep running right you will find two Ring monitors and a 1-up monitor.