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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."
Contributed by KnowledgeBase
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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.
Contributed by SuperFlorian12
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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.
Contributed by SuperFlorian12
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The Chaos Card is actually the card "Chaos Orb" from Magic the Gathering, both in appearance and effect.
Contributed by Mass Distraction
The Fortune Telling Machine has a chance of giving references to other games and movies, such as the line "Your Princess is in Another Castle" from Super Mario Bros. and "Look to La Luna" from Mortal Kombat.
Contributed by ThisGuyInTheSuit
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.
Contributed by Mass Distraction