A packet of cards is introduced for a "kind of" poker game in which the spectator cannot lose. The magician explains he has two rewards for him. At the end of the dealings, if the spectator's hand beats the magician's, he gets a $20 bill. If his hand is a royal flush, he gets the magician's credit card. And if the magician's hand beats the spectator's, nobody loses anything. Fair enough?
The cards are then dealt accordingly to the spectator's choices between the magician and him - five cards each. When the spectator spreads his cards face up on the table, he has four cards from a royal flush in Clubs (10, Q, K and A) and a Joker.
The magician explains that a Joker usually replaces any card. But he reminds him it is just a "kind of" poker game and asks him to read what is written onto the Joker. The card is removed and he reads, "Guarantee to replace any card except the Jacks".
But there is more! Maybe the magician didn't put any Jack into the packet so the spectator could not win? The magician shows his cards to reveal he indeed got the missing Jack... Actually, all his cards are the Jack of Clubs!
Uses only regular Cards. No fake, no sticky cards. No sleights. The spectator can manipulate his cards as he wants (they are normal) and the Jacks can be closely examined.