The underside of a hero

Not even the Special Stage sprites are exempt from historical oversights. Aside from a really bad misalignment with the standing frame, each sprite features a highly-detailed head that uses a bunch of different colors, whereas the torso and the legs seem much rougher in comparison, using just two shades of blue, at times resembling basic 3D polygons.

It would appear that once again, some of Sonic's sprites were left half-unfinished. But that isn't the only oversight these sprites share with the monkey bar animation.

In both Sonic 1 and Sonic 2, the soles of Sonic and Tails' shoes were colored the same as the rest of the shoe: red. For Sonic 3, the developers changed it so all three characters have black soles, but this redesign was never reflected in the Special Stage sprites for any of them.

Given their close tie to Sonic 2, it should be no surprise that Sonic's monkey bar sprites also feature red soles. They're not alone, though: red soles are also present in some rotation sprites used in Hydrocity Zone, Mushroom Hill Zone and Lava Reef Zone, as well as the horizontal "corkscrew" animation, which is lifted directly from Sonic 2.

EDIT: This was pointed out by commenter greenknight9000, which I somehow missed the first time around. My bad!