The Invisible Deck is a good "out" for many card tricks. If you lose track of their card, don't know what it is, just shuffle a couple of times, cut the card and reveal -- the wrong one. (Or you may get lucky at this point an land on the right card. Take a bow and don't let on that you couldn't do it again to save your life!) If it is the wrong one, ask the spec to name the card. Whip out the Invisible Deck, say, "Last night while I was practicing, I had the weirdest feeling about that very card! So I put it in this deck, face down. It must have been a premonition that you would pict that one particular card. Here, I'll show you," cleanly show that their card is the only face-down card in the face-up deck, and you've recovered!
You can't do that too often, and not more than once in one show, so practice until you don't mess up. Actually, you should always practice until you can't get it wrong before performing. Then you'll only need this escape clause in really rare events that are beyond your control.
Robert V Frazier