Too tall Knuckles

For the most part, Knuckles' sprites in the cutscene art block are identical to his player sprites from Sonic & Knuckles. Apart from differences in the color palette, the only other changes are to his running animation: Knuckles is two pixels shorter in Sonic & Knuckles than he was in Sonic 3.

In the process of lowering Knuckles' center of gravity, they also took the opportunity to properly paint his fists over the sloppy remains of Sonic's stomach, from whence the sprites were obviously edited. But not over the tan pixels around his fists, though, that would've been far too classy.

And if that's still not enough class for you, they actually managed to keep the original Sonic 3 frames around somehow. The pre-rotated versions of the running sprites were made before these changes, and since no one bothered to update them after the fact, Knuckles gains an inch whenever he runs up walls. Must be the lighter load on his legs.