This effect, if carried out correctly, will shake anyone's confidence in their logic as well as in their memory!
Flash-out your credit card, and your video membership card(V.M.) Display them both overtly, front and back and then neatly withdraw the credit card and place it behind your back. Meanwhile stretch the hand holding the V.M. card facedown toward the volunteer and challenge him for a quick memory test. The question is as you would expect it to be is;"Which Card am I holding?" And the anticipated answer unless the volunteer has a serious memory problem, would be the V.M. card. "Sorry maybe next time!" Turn your hand and open it to reveal the face of the credit card to the astonished crowd. Then just to add a little salt, you bring back your other hand from behind your back to reveal none other than the V.M. card.
The great thing about this effect is that it can be carried out many consecutive times, each time further shaking the volunteer's confidence. The irony is that he'll keep asking for one more chance.
A mind boggling effect that will simply leave them wondering!