From Bulbapedia, the community-driven Pokémon encyclopedia.
Card Pop! is a feature in the Pokémon Trading Card Game and Pokémon Card GB2: Here Comes Team GR! video games that resembles Mystery Gift. When the infrared ports of two Game Boy Colors with the game are connected, each player will receive a random card. Card Pop! is the only way a player may receive a Phantom Card (Mew and Venusaur in Pokémon Trading Card Game; Lugia and Here Comes Team Rocket! in Pokémon Card GB2). However, a player cannot Card Pop! with the same person twice.
While Pokémon Card GB2: Here Comes Team GR! also includes Card Pop! as a feature, each game in the series is only compatible to Card Pop! with another copy of the exact same game. Attempting to do Card Pop! between a copy of Pokémon Trading Card Game and Pokémon Card GB 2 will result in various glitches, including a game freeze or possibly losing saved game data.