Sewer door deletion
Sewer door deletion, or throne door deletion, is a major glitch that removes the shutter door on the back of the throne in Hyrule Castle. It's usually done as a prerequisite to dungeon ID swapping.
You delete this door by copying the state of an open door in Tower of Hera over it. Deleting the sewer door is an effect that persists when you save your game.
It can be accomplished by two methods: using the Somaria Door glitch, or using two bottles for two YBAs in a row.
This is the door we're deleting
It still looks the same, but you can walk or run right through it
Somaria method
Two-bottles method
Bring two bottles to Hyrule Castle so you can do two subtile YBAs. You won't have an opportunity to take damage or use magic in between, so the first bottle has to be red, so it won't refill your hearts and magic and make the second YBA impossible. The second bottle can be red or green.
Make sure you're not at full health. Hold out your sword and move sideways into the door until you're on the bottom pixel you can be on. On the Link sprite, two rows of Link's hat are visible.
Equip a red potion, and press down and Y simultaneously. The screen will fade to an ellipse that's mostly off the bottom of the screen.
A side effect of a downward YBA is to Door Juke you past the door transition, so now you need to back up to line up for the second YBA.
Carefully tap left or right until you reach this position. You can't see Link, but you can tell by the way the camera moves. A row of lighter pixels on the doorway -- the second from the bottom of the screen, which is the third from the base of the door -- should be the bottom pixel on your screen. If you overshoot by one pixel, it's okay.
Do a YBA again -- select a red or green potion, and press down and Y simultaneously. The screen fades out again.
You won't be able to see the effect, but if you successfully did the two YBAs, the sewer door will be deleted. You can mirror out or save and quit, and go on to do the glitch you're deleting the throne door for, which is presumably dungeon ID swapping.
Unfortunately, you only get one chance at the second YBA.
