Sonic the Hedgehog is an SMF2 hack made by Michael. It's SMF2 2.1 except the characters, sprites and tiles are from various clashing sprite sheets from interactive audiovisual entertainment in the Sonic the Hedgehog franchise.
The hack was being worked on at the same time as Crash Bandicoot Edition, and as hacks were still a fresh, new, exciting concept, the hype of one would definitely take away from the one after, so the hacks were rushed to hell in order to get one out before the other. Sonic the Hedgehog's first version was released a while before Crash Bandicoot Edition, though the rushing, combined with the problems of adapting an SMF hack to another franchise, caused both to be garbage, but Sonic the Hedgehog is the less garbage one. Sonic the Hedgehog also used hex editing to change its text, although with varying degrees of outcome due to the limitations.
A demo version was given to luigibonus (ironically the creator of CB Edition) and a few others a day before the public release. The only difference was forgetting to change the cape feather and plastering 'DEMO' over the screen in cooltext.
There is medium demand for a Ver. C version of the "Sonic Flash," to which the creator has responded by doing nothing. However, he has stated he has hopes to use the colour palette feature to alternate between the 'hill zone' and other tilesets throughout the different games (Green Hill, Emerald Hill, Your Mum Hill etc), despite not wanting to go through that development hell ever again. He also feels that all the changes necessary to make it good would give a lack of backward-compatibility with the previous version.
Actually, some effort was made by the creator to make a (still in 2.1) version of Sonic Flash which used shape editing, however PG was dying at the time so it joined the "Hacks that Didn't See The Light of Day" club.