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.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.In programming 'Undefined' refers to an unknown value.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. 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.

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.

Wisps fire tears with Playdough Cookie effects.

Book of Virtues: Creates blue glitched wisps on the middle ring.

The Error room will always contain a trap door, not a lightbeam, so it will always send the player to Sheol.

Using Undefined on ? always sends the player to the I AM ERROR room.

This can be a reliable method of gaining many items on the way up to Home, while also allowing the player to skip potentially difficult encounters.

As no Secret Rooms or Super Secret Rooms spawn on any floor during the Ascent, this item will only warp Isaac to either the Treasure Room or the I AM ERROR room.

The Void is also reset and can be cleared repeatedly.

If Isaac has an item that resurrects him as a different character, multiple achievements and items can be unlocked in a single run. This makes it possible to kill ? or The Lamb multiple times.This can provide a ridiculous number of items from the golden chests in Chest. Chest and Dark Room are reset on exiting the I AM ERROR room, which can be done repeatedly as long as the player can gather enough charges to successfully warp to it.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 The Polaroid or The Negative.The chances for each main destination are equal.There is also a minuscule chance of teleporting into a Black Market. Teleports Isaac to the Treasure Room, the Secret Room, the Super Secret Room or I AM ERROR.

Binding of isaac items rebirth