I AM ERROR is an extremely useful, but very rare special room that can be accessed by using the Telepills. There is a 6.66% (1/15) chance of teleporting to this room. Contrary to popular belief, the I AM ERROR room cannot be found by using Teleport! or Tarot Cards (as of the original v1.48). It can, however, be accessed using Teleport! in the demo. In Rebirth, the Undefined item has a fairly high chance of taking you to this room, among other useful rooms.
Inside, the player will find a flashing and twitching figure that resembles the shopkeeper, but has a black forelock and a beard, as well as a violet shirt, saying "I AM ERROR." The room background will constantly flicker and occasionally flash to one of the possible backgrounds of the chapter the player is currently in. The walls appear to be bigger than average room walls, and are completely black, making the room seem like a real error.
The room will have a trap door which takes you straight to the next floor. The trap door is the only way out of the roo, unless you use the item Teleport!, Telepills or any tarot card that has a teleportation effect (The Fool, The Moon, The Stars, The Hermit, The Joker or The Emperor.) Keep this in mind when you use pills, so you don't accidentally teleport yourself out if there are still identified pills or other items aying around.
The room can contain the following:
A random free item and 9 random pills.
A random free item and a lot of flies.
A random free item and 4 (unlocked) treasure chests.
A random free item, 13 coins, and 3 priced things that can be items or pickups.
This includes items that cannot usually be bought in the shop. (Whether any item or just a broader array of items is available needs confirmation.)
The appearance of the figure in this room is heavily based on this character, and might even represent him.
This room tends to lag due to the constant texture changes on the floor.
Getting teleported to the Error Room in The Chest will keep you trapped, since there will be no trap door to leave. It will appear as a Regular Room with The Chest backdrop, but there will be no doors to leave or any trap door.(Fixed in afterbirth.)
You can get to The Womb via the Error Room without having beaten Mom; this might be unintentional. The same is true for Mom's Heart, although early access is possible here by normal means through the Devil Room.