Does your thumb regularly slip off the Circle Pad when you’re playing the Nintendo 3DS? I don’t think I’ve ever had that problem, but it sounds like the type of thing that could happen. And, if it’s something you find that annoying you may want to pick up a Circle Slide Pad Cover. These things pop onto your Circle Pad and change its texture, making it easier to grip.
You can click here to view their sales page, although it’s all in Japanese. I’m not sure if there’s any English site you can purchase them from.