Clipping through floor on Emerald Coast

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I have a problem on the "Emerald Coast" stage in Sonic Generations on the 3DS. On one hill I seem to just clip through. This is fine with modern Sonic, but only occurs with classic Sonic, thus not letting me progress in the game.
What I want to know is this a game-breaking bug or just a glitch? I payed 20 pounds for this and I might return it.
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Images: Константин, Pok Rie, Pok Rie, Tholaal Mohamed