Setup: Place a card under your bottom (=ass).
Sleights: Deluxe Rub-a-Dub-Dub, Classic Palm, One Handed Pop Out, Spring.
Performance: Have the spectator select a card. Vanish it. Show how it travels through the hand, up the sleeve, to the shoulder, then... then... what’s happening? - the card is not going the right way! Instead of going to your other sleeve, it’s going slowly down the neck #swallow, then down the tummy and finally you eject it through the...the...ass.
Produce the card from underneath your ass, but don't show the face of it. Apologize for the error. Hand the card out for smelling. Bring it back after being refused. Take the card behind your back and pretend to insert it back where it came from and Classic Palm it.
Gesture how the card now travels up the stomach, through the lungs, left shoulder, down to the elbow and finally out the sleeve, at which you pop their selection (which is on the top of the deck, face down) using any preferred technique.
You can now repeat the ruse (remember, you have a palmed card in your right hand). Drop the face down "X" card on top of their selection and vanish the card once more (this time a lot more cleanly). What you do from here is up to your character and imagination.
Ideas
If you placed a card in the spectator’s pocket or underneath him as he sat down, you can hand him the invisible card and have him perform the same actions/gestures.
You could have the card accidently pop from somewhere else, for example like your mouth (of course, with the card still colored brown). I already see an entire act build around this idea, don’t you?
Instead of having cards underneath you, have them in your back pocket. Have more than just one. If your character is more Jerry Sadowitsy, have a small packet between your butt-cheeks and vanish an entire packet then pull your pants down and let the waterfall of cards dazzle the spectators as they wonder what is happening #Waterfall-Ass-Spring. Leaving the joke aside, you could achieve this Ass Spring by palming a packet of cards and springing it from the side of your butt (I've done before).
Horror Sketch
Concept: A magician teaches people how to do different vanishes, only to find his students in excruciating pain after performing the sleight
Script: We see the magician teaching a kid how to vanish a card and produce it from his pocket. The kid vanishes a card then falls down to the floor, yelling out in pain, while the card is seen travelling visually through his arm, down his throat, out ripping his skin and land in the inside pocket of his jacket.
Time goes by and the magician forgets of this incident. At a party, someone asks him to be taught how to vanish a simple object as a pencil. The magician teaches the person, then, upon having vanished the pencil, the spectator yells uncontrollably as people around them see the pencil travel through his skin, with no apparent destination.
This time the magician figured out he should never teach anyone else how to vanish objects. Time goes by, months and years, and the magician is seen at a family party. One of his nephews is performing a magic show for the family. The magician smiles and in the excitement of the moment, forgets of the past. After the show, he approaches his nephew and gives him a couple of tips on producing the coins from thin air without flashing. The boy mimics his uncle's motions, producing coins from thin air one after the other. The uncle gasps flabbergasted at this small miracle. It is obvious that the boy is not performing sleight of hand, but actually producing coins from the air. We see the boy running towards his mother to show her how he can produce coins from thin air. Fade out as we hear the mother praising her little magician.