[CORed]

I'm assuming you're talking about a situation where you notice a marked card and decide to take advantage of it. The card may have been marced intentionally or accidentally creased or whatever. If it's an ace, knowing this card could give you and edge. The question is, has anybody else noticed it? Did someone mark it deliberately? The safest thing to do, as well as the right thing to do, is to tell the dealer and ask for a new deck. This is why players are allowed to ask for a new deck. Unfortunately, most of the requests for new decks come from idiots who hope that a new deck will change their luck.