TST Torn Seal Transform by Justin Sopher

A torn corner is shown inside a plastic bag, its identity unknown. A card is chosen by the spectator and lost in the deck. The magician takes the bag, shows the corner inside it to be blank. The spectator names their card and with a flick, the corner transforms into a corner of their selected card inside the bag.

When the spectator looks through the deck to find their card, they see it has a corner missing... and of course it matches perfectly with the one in the bag.