The after credit illustration card features Modoka, Mami, and Sayaka. If you reverse the card it forms the Kanji character for death.
Kyouko playing a Dance Dance Revolution-like-game, to the tune of the ClariS song, "Connect", the opening theme. Several months later, Dance Dance Revolution X3 was released, and it features a cover of "Connect."
Madoka's keyboard is similar to existing commercially available Laser Keyboards.