未定义#324

Undefined is an unlockable activated item.

• The item is a reference to a bug in the original The Binding of Isaac, where exhausting an Item Pool (or rarely simply by chance) would cause an item to spawn whose name and description was listed as "Undefined", and whose sprite would rapidly cycle through the sprites of every other item in the game. Undefined by itself creates no effect, but if it was ever picked up, should the game attempt to spawn an item from an exhausted item pool, the game instead crashed, as it could no longer spawn Undefined. From Rebirth onwards, an exhausted item pool will instead spawn Breakfast, even if the item would normally never spawn from that item pool and/or Breakfast had already been obtained in the run.
• In programming "Undefined" refers to an unknown value.
• Given the Glitch theme of the item and the ability to access Chapters that have not yet been unlocked using I AM ERROR, along with the ability to acquire mass upgrades in The Chest and The Dark Room, it is possible that it was intentionally made into a powerful sequence-breaking item.
• The sprite for Undefined appears to be, fittingly, a glitched and spliced version of other item sprites, with the bottom right part noticeably being the The Compass, the bottom left being the Blood Bag, and the top half being Contract From Below. This may also be a reference to Undefined's sprite in the original The Binding of Isaac.