Missing No.

Missing No. is an unlockable passive item.

Effects

See D4 for more details of the rerolling process.
Due to losing all items every level, it is beneficial to try to gain permanent stat boosts if possible. These include using pills when equipped with PHD or Virgo, sacrificing health with Potato Peeler to gain damage or Brittle Bones to gain tears, or using up Judas' Shadow.
Like other full-reroll effects, Missing No. has a small chance to not reroll every item.
Can cause transformations.
- Transformations are lost when rerolling out of the items that caused them.
Missing No. cannot be re-rolled or removed by most means after being picked up, including D4, Clicker, XVII - The Stars?, or taking damage as Tainted Eden.
This item can be used to gain more health if desperately needed, as the randomization does often include items that grant you various types of hearts.
If the player has already obtained a large number of items (such as during a run where they have gained infinite money in greed mode), it is highly recommended to avoid this item, as it will almost always end up rerolling all the items the player has thus far collected into Breakfast.

Butter! / Genesis: Provide the only available methods for getting rid of Missing No.
Glowing Hourglass: Using Glowing Hourglass to go back to the previous floor will keep the rerolled items, which will allow to reroll them again.

The Stairway: When getting the Stairway at the start of a floor, the ladder doesn't spawn. It will spawn at the start of the next floor even if rerolled.

This item is a reference to the infamous MissingNo. glitch occurring in the first-generation Pokémon games. The glitch would often corrupt save files, and therefore randomize various parts of the game as it would use the incomplete index values attributed to entities not included in the final release of the game. Isaac's appearance when affected by Missing No. is based on this glitch which could cause the player's sprite to become scrambled.
The item also resembles the painting "Composition II with Red, Blue, and Yellow" by Dutch painter Piet Mondrian.
The background color for the Missing No. achievement has the same color as all the challenge unlocks. This implies that Missing No. was originally intended to be unlocked via a challenge. (Or, perhaps, it was intended to be looked at as an error.)
- The background color for the Suicide King achievement has the same color as all the boss rush unlocks. This implies that potentially Lazarus would have unlocked Suicide King for completing the Boss Rush and Suicide King (challenge #7) would have unlocked Missing No.