Touching Ultra Greed while carrying Midas Touch will greatly reduce his armor and will make him take extra damage on top of the usual effects, in reference to the fictional story of king Midas.
The iOS release of the game was rejected by Apple from being sold on the App store due to, according to a notice: "contains content or features that depict violence towards, or abuse of, children, which is not allowed on the App."
Owners of Rebirth got a discount of 39% for pre-ordering the Afterbirth DLC on Steam. This dropped the price from $10.99 to $6.66.
The Japanese release is called The Legend of Isaac (アイザックの伝説) and the title screen is a reference to the Japanese title screen of The Legend of Zelda: A Link to the Past.
The Chaos Card is actually the card "Chaos Orb" from Magic the Gathering, both in appearance and effect.
Spoiler:The Boss Rush Room that can be accessed by getting to and beating Mom in The Depths or Necropolis within 20 minutes consists of (nearly) every regular boss in the original Binding of Isaac and Wrath of the Lamb.