Clipping through floor on Emerald Coast
Doing a quick look online, I found this YouTube video, which I think might be what is happening with your game.
A bit of research tells me that Sonic Generations on the 3DS did not receive as good reviews as it did on other consoles, but it still was not picked up for being unplayable.
That said, if you are experiencing bugs (especially if it is making the game unplayable), UK consumer law should entitle you to a full refund or exchange.