Shadow gives over the fourth Chaos Emerald in this game, a possible tribute to Shadow's memetic line, "Where's that damn fourth Chaos Emerald?" from his earlier spin off title, Shadow the Hedgehog (2005).
Like the Wanted Poster bearing the cameos of several characters, there are also two Missing Posters of Mighty the Armadillo from Chaotix (1995) and Ray the Flying Squirrel from SegaSonic the Hedgehog (1993). Both posters say "Missing since 1993."
Crisis City was erased from the timeline after the events of Sonic '06. This means that Sonic should not remember that level at all, and that the level should not even exist anyway, as that future was prevented from occurring.
There were plans for ports for the Nintendo DS, PlayStation Portable and Nintendo Wii.
20th Anniversary project for Sonic the Hedgehog.