Undefined

Undefined is an unlockable activated item.

Effects

 * Upon use, teleports Isaac to the Treasure Room, the Secret Room, the Super Secret Room, I AM ERROR or Black Market (very low chance).

Strategies

 * The I AM ERROR room in Cathedral and Sheol will have a beam of light/trap door leading to the next floor, even if Isaac doesn't have or.
 * The Chest and Dark Room are reset on exiting the I AM ERROR room, which can be done repeatedly as long as the player is able to gather enough charges to successfully warp to it. This can provide a ridiculous number of items from the golden chests in The Chest.
 * This makes it possible to kill ??? or Lamb multiple times. If Isaac has an item that resurrects him as a different character, multiple achievements and items can be unlocked in a single run.
 * The Void is also reset and can be cleared repeatedly.
 * As there are no I AM ERROR rooms in The Chest, the Dark Room and Ultra Greed in Afterbirth, these floors cannot be cleared repeatedly.

Trivia

 * 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 were listed as "Undefined", and whose sprite would rapidly cycle through the sprites of every other item in the game. Undefined by itself had no effect, but if it was ever picked up, should the game attempt to spawn an item from an exhausted item pool, the game would instead crash, 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, noticeably the bottom right part being 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.
 * Once all the items have been exhausted in a reset Chest run, chests will only drop Breakfast HP upgrades.

Seeds
TRP0 FE16 (first treasure room, adjacent to spawn) 9YDH V6SL (first treasure room, adjacent to spawn) F189 W42E (first treasure room, adjacent to spawn. Teleports to I AM ERROR room upon first use in hard mode.) 0JGY 6ZWX (first treasure room near spawn) W0WZ QCRJ (first treasure room, adjacent to spawn) 6AGC GCCH (first treasure room, adjacent to spawn) SJKJ XZB2 (first treasure room, adjacent to spawn) 4CG9 42ED (first treasure room, adjacent to spawn) YAD2 KJCM (first treasure room, adjacent to spawn) 8LQF 7MT0 (first floor treasure room)