Pick a card. They look at the card and memorise it. While they do that, you flip the deck over and then flip the top card over again so that it looks like a regular top-down deck again. You tell them to slide the card back in anywhere. Shuffle it as much as you want. Go through the deck and take out the card that's flipped the wrong way (the one that you didn't flip over twice o'course).

Lame trick and they can probably work it out when they notice that the card is flipped over.