Sonic 3D: Flickies' Island/Hidden content
From Sonic Retro
PC Version
Missing Render
KojiChao found what appears to be a piece of concept art piece for the final boss, possibly used as part of an 'explosion' sequence after defeating the boss. The background is very dark and can barely be seen, unlike the Sonic Crackers-esque backdrop seen in the final game. The boss is also equipped with items from its third and fifth phase.
Unused Special Stage Intros
These images may have been meant to be used for loading screens shown before the special stage. However, since the Special Stages don't require much loading time (they appear instantly), the images went unused. Perhaps this means the Saturn special stages were originally going to be used in the PC version?