That's cheating, but I'll let it slide

Reader Silver Sonic 1992 asks:

A bit unrelated, but is there a reason there is a duplicate of the Sonic's falling animation?

First, a little background. All three characters have three distinct animations that make use of their falling sprites. These animations take up IDs $19 through $1B. Here are the definitions for Sonic's animations:

byte_12C0A:     dc.b    9, $D7, $D8, $FF

byte_12C0E:     dc.b  $40, $8D, $FF

byte_12C11:     dc.b    9, $8C, $8D, $FF

The second one, $1A, is the animation which plays when falling back from taking damage, and consists of only a single frame. The third one, $1B, is used at the start of a few levels, and alternates between the damage frame and a second one to show Sonic flailing his arms as he falls into the stage.

Now the third animation, $19, is used while sliding on stuff like the icy slopes in Icecap Zone 1, and the sand streams in Sandopolis Zone. As it turns out, if the regular falling animation is used for these obstacles, Sonic's sprite actually floats a fair bit off the floor:

The developers' solution was to duplicate the mapping frames used by the falling animation, shift them diagonally down towards the slope, make a copy of the falling anim that points to those frames, and have slides use that anim instead:

Not all slides, however. If the water slides in Hydrocity Zone 2 used the proper sliding anim, you wouldn't be able to see much of your character due to the high walls of the waterway:

In this instance, the developers cheated and made the water slide call the falling anim instead, which means that Sonic is technically hovering in mid-air again.

And that's why there's a duplicate of Sonic's falling animation. Unless you were talking about mapping frames $D0 and $D1. I think those are just unused.