The immediate result is that you'll hear the mirror noise with no change on the screen, and in fact the game will seem to freeze for a while. You can make sure that you're not frozen for too long a while, and then get various possibly useful effects.
Before you do this glitch, it's very important to pick up a pot on the same supertile. This sets a value that will be used as a counter. Mirror Door is not worth attempting on tiles without pots.
Do not press select or X after picking up the pot, or you will reset the value. If you need to buffer, use the start button instead.
The glitch requires using the mirror inside a door, so you need to be in a position where you're allowed to use the mirror. For one thing, this means you can't be in cave state.
You can't usually use items in a door, but you can if you've snapped into the door. You can do this by using a bomb snap or the Cane of Somaria to get you to the furthest position inside the door.
The effects of Mirror Door are to "open the triforce door" and perform another effect that depends on the direction of the door.
"Opening the triforce door" means the following will happen:
- All the action in the game (including any actions by you) freezes for an amount of time that depends on the global Y-coordinate of the pot you picked up. You'll usually have to wait for 30 seconds to 2 minutes. If you didn't pick up a pot, or if you pressed select, the wait will be an impractical 18 minutes.
- A weird-looking door will open up in the northeast quadrant of the room. You can walk through it.
If the door faces south or east and isn't an auto-door, or in any kind of west door, the additional effect will be a Door Juke. If the door faces north, the effect will instead be similar to a north subtile YBA.
The camera will usually end up misaligned as a result of the Mirror Door glitch, so some screen transitions afterward will have unusual results.
Example: North Mirror Door from Desert to TT
This glitch gets you access to Thieves' Town (NL) from the back hallway of Desert Palace (NL), and can be a useful way to access TT and the Village of Outcasts before you have a bottle. You won't be using the "triforce door" that opens up, because it leads into a solid wall; instead, you'll be using it for the subtile YBA effect.
Don't back out of the door you're in after the Mirror Door effect happens, or you will de-spawn sprites, including crystal switches. Just go directly through it.
The camera will be weird and unlocked after you enter Thieves' Town. Using the stairs a few times will partially, but not completely, fix the camera. If you try to go south on the conveyor belt (the "Hellway"), you will probably wrap the screen and get softlocked. You will also softlock if you try to walk north through a subtile door.
There's still a route you can follow through the dungeon, which starts with going down the stairs, collecting the key, hitting the crystal switch, and returning upstairs. You can bomb the floor, and you can reach Blind's cell if you have the big key.
At the cost of a small key you wouldn't normally spend, you can open the Big Key Door from behind, and get out to the front of the dungeon and the Village of Outcasts, or you can also get there by defeating Blind.
Use a bomb or the Cane of Somaria to snap into the highest pixel of the door. Then select the mirror, and press up and Y on the same frame.