I think that the most accurate answer to your question would most likely come from someone at NGC. Go here: http://www.ngccoin.com/index.aspx then click on "contact" at the top of the page.
Not sure I've heard of this happening. The white insert in the NGC slabs is a non-slip surface, feels like it's made of a coated rubber material. But I agree with rick, NGC would better answer this one. You can call them at 800-NGC-COIN.
They will fix it, but you will have to pay for shipping both ways if I understand it correctly. So you might want to do it when you send in coins next time. This happend to me as well and they fixed it.