Mecha Sonic in Sonic the Hedgehog
From Sonic Retro
Mecha Sonic in Sonic the Hedgehog (aka Mecha Sonic in Sonic 1) is a hack created as a proof-of-concept by Iron Sonic of Sonic the Hedgehog for the Mega Drive in which the player plays as Mecha Sonic from Sonic & Knuckles. The controls are the same as in the original Sonic 1, save for the ability to perform the Spin Dash. Each sprite has been edited, and the player size and other aspects of the Sonic object have been changed to accommodate Mecha Sonic's larger size.
Whilst the game is mostly playable, there are certain areas in the game that Mecha Sonic cannot complete due to his increased size; prime examples of this include Spring Yard Zone and Scrap Brain Zone.
These are the zones that are known to be 100% completable with Mecha as of the current build:
It was planned, that in a future revision, the level layouts would've been slightly altered to make Mecha Sonic become 100% playable in all levels. In addition, Mecha Sonic would've been planned to have his Super form become accessible by collecting all the Chaos Emeralds.
Iron Sonic himself has not posted on Sonic Retro or participated much in the Sonic scene lately. In mid-2010, he asked RGamer2009 to determine if there was still interest in continued development of his hack; numerous enthusiastic responses led to development being resumed. Development was still ongoing as of February 2011, as confirmed by E-122-Psi, who also presented a new screenshot courtesy of Iron Sonic. It was revealed to be cancelled sometime later.