A user at The Sonic Stadium, a Sonic the Hedgehog community and forum, noticed that a beta version of Sonic the Hedgehog 4: Episode 2 was available to download through Steam earlier today. Though the leak has since been plugged its resulted in some additional screenshots of the game and early user impressions.
Jumping feels better—"you don't lose all forward momentum when you jump," wrote one commenter who played it, "and rolling does pick up speed down slopes (but not nearly enough, only really steep slopes). [Y]ou can definitely tell this is Sonic 4." Other comments have praised the game's graphics and sense of humor.
It was, however, an incomplete build of the game, so it had bugs, missing textures and other issues you'd expect of a beta. Sega was notified, wiped all discussion of the beta from its official forums, and ultimately got Steam to pull the beta from the store.
Those screens above all came from The Sonic Stadium. Additionally, someone captured video of a cutscene from the game, so that merits a spoiler alert. If you still want to see it, it's here.