Ashura
From Sonic Retro
Ashura is the fan-atoned name of the glitched-palette phenomena found in Sonic the Hedgehog 2's Emerald Hill Zone.
Getting Ashura
In order to reproduce the Ashura glitch, the zone select and debug codes must be active. Upon activation, proceed to one of the two acts of Emerald Hill Zone, as it does not occur in any other level. When the zone begins, press 
. Sonic will be a ring, signifying that debug mode is active. Press 
four times; Sonic should be an invisible object. Then, press 
repeatedly to create a number of waterfall objects. Press to change back into Sonic, who, if the glitch has been activated correctly, will emerge from behind the waterfalls with a green and black palette.
Explanation
The reason for the Ashura palette glitch is a result of several objects being loaded into the Sprite Status Table at a given point in time. Since RAM on the Mega Drive is in limited quantity, only so many objects can be loaded into the status table before it reaches its alloted limit. When a mass of waterfall objects are placed using debug mode, this overloads the SST to a point that exceeds the SST's reserved RAM area; since the palette cache used by the game to update CRAM is located after the SST in system memory, SST data becomes parsed as color data in the reserved palette cache area and is loaded into CRAM accordingly. Because of this, the green color is purely a coincidence--it is not intentional.
Similar palette errors can occur upon porting the Sonic 1 sound driver to Sonic 2; if Sonic loses rings whilst underwater, it will overload the SST and spill into the palette cache, corrupting the underwater character palette line.
Origin
Kushami, a member of the Sonic Retro forums and a veteran of the online Sonic scene, posted the following first person explanation of the origins of the Ashura glitch: <ipbwiki_post post="203476" />
Miscellaneous
Though only a palette glitch, Ashura is one of the four characters possible in Yuski the Dog's SMS ROM hack, Sonic vs. Os Mauniks. Ashura was even hacked into Sonic R [1] and Sonic Heroes [2]. Also, there is a way to get an "Ashura"-esque Chao in Sonic Adventure 2: Battle; it is the dark/run Shadow the Hedgehog Chao. Additionally, Ashura is the titular co-villain in the fan-made game Ashura Dark Reign.