Dude as I Do King of Clubs by Liam Montier

King of Clubs - Write your own prediction on the card case (looks 'organic')

Ten Of Hearts - Prediction is pre-printed on card case (it's the standard 10 of Hearts that is on the normal Bicycle case)

You take 26 cards from one deck of cards, and your spectator gets 26 from a different deck. You BOTH shuffle your packet. With your cards behind your back you both pass a random card to each other, and slip it into your half deck.

When the packets are then spread face down it is clear that there is indeed one (and only one) odd backed card in each packet. When you flip them over you discover that they match. Both of you selected the Ten Of Hearts.

'Hang on', your suspicious spec proclaims, 'Maybe ALL THE CARDS ARE THE SAME'.

'Oh, they are!' you agree, 'But how would that help?'

And then you flip all the cards over (yours and theirs) to reveal that all the remaining pasteboards are duplicates of the Six Of Clubs. You both picked the ONLY Ten Of Hearts in the other person's packet!