First of all, go to this room in the Skull Woods dungeon, then situate Link between these two bumpers. Let him rapidly bounce between them for about a minute (and be sure to keep your sword charged to eliminate the hand when it falls.)
If it worked, when you leave through the upper door, you'll notice that you can see more than one room at a time. Normally, you can NEVER see beyond the room you're currently in. The game's camera has essentially become dislocated and there are many amazing results of this glitch...Start by going through that door on the left. If you got Version 1 of this glitch, the room will scroll twice instead of only once, and you'll become stuck in a wall. If you're lucky enough to have gotten Version 2, the room will scroll in only once and appear normal. If the room only scrolled once, then turn around and leave through the door you came in.
Now here's where things start getting strange. You no longer coincide with your surroundings. Here I am standing over a bottomless pit...
And here I fall into a pit that can't be seen. There's a reason for this. The problem is that the game thinks you're in another room, and in fact, you actually are. What you're looking at here is a "Phantom" Link. The real Link, whom you're actually still controlling, is in the room the game thinks you're in, and thus this Phantom Link is experiencing what's happening to the real Link.
Phantom Link, you'll notice, doesn't have a shield...Let's try to scroll the real Link back into view...
By going towards the edge of the screen, the real Link's shield suddenly becomes visible on the other side.
Walk off-screen and the real Link pops into view. Phantom Link has disappeared. Now go back through the door, and...
Wow. We've warped completely into the next part of the Skull Dungeon, beyond where you're supposed to use the Fire Rod! As you can see, that's the Phantom Link (no shield) again. So what apparently is happening is that the real Link is going into the correct room, but the camera is scrolling beyond where it should. I have no theories as to why that results in a Phantom Link as opposed to something like that Dracula X glitch where the elevator leaves without you and then the screen is just frozen that way.
Several side effects of this glitch include invincibility to enemies. Some traps still damage you (like spikes), but most enemies, like these mini-moldorms and fire chains, can be walked right through with no consequence. Also, most of your items become unsuable. Try to throw the hookshot, and there's no chain. Arrows don't work. Neither does the Cane of Somaria. If you place a bomb, you won't see it, but it will still explode after a few moments. Pick up a pot, and it disappears, though Link's arms are still in the "holding it over my head" position. If your hearts are full, you'll hear the sound of your sword's beam when you swing it, but it doesn't actually fly off. Sometimes, you can scroll through doors in such a way that Link ends up UNDERNEATH the floor. If that happens, all sorts of crazy avenues open up...
Now, if you can get Link underneath the floors (you can see him only when he passes under bottomless pits, or by removing the background layer on an emulator), you can go scroll into many different locations of the game...
The game contains many blank rooms of this same shape and size. There are no doors or other features. They apparently were used to fill up space in the memory.
There are also blank rooms of this size and shape. Sometimes it seems they act as "separators" between the dungeons, since all the dungeon rooms in this game are actually on one continuous map.
Holy shmokes! I've scrolled from the Skull Woods dungeon into a room of Ganon's Tower!!
Take a look at the map. That's still Skull Woods Dungeon, but the game doesn't know where you are.
A room in Ganon's tower, but the graphics aren't loading correctly. Also, since the screen scroll is free-flowing, I scrolled that mini-helmasaur enemy into the bumper and he got stuck there.
In the middle of Ganon's Tower is stored....one of the Death Mountain caves, but it's palette is messed up (the floors look like Ganon's Tower.)
This is a fairy room from Ganon's Tower, but its palette has taken that of the caves.
Made it to the entrance of Ganon's Tower, but the Pig Statues didn't load correctly. They appear as Fairy Statues instead. (Going up or down the stairs and then coming back to this room corrects the graphics.)
After scrolling around some more...wow! Lanmolas appearing in the wrong room!!!
This was interesting. If I moved close to the Lanmolas, they'd freeze. To get them to move again, I had to walk over close to the sandy-floored room that they'd normally appear in.
After I left this room, I got stuck with no way to move on. It seems like you can only scroll so far before that happens.
There are a lot of different things you can do with this glitch, most of which are already documented at David Wonn's Unique Glitches Site, but I found something unusual that I've never seen mentioned anywhere else:
An unused dungeon tile that's hidden somewhere in the ROM's tilesets. I don't recall seeing that giant skull anywhere in the game. Since these glitched-up levels are pulling graphics from the Skull Woods dungeon area, I'm guessing it was originally intended for use in that level.
UPDATE: Here's something interesting I noticed when I was replaying The Legend of Zelda: Link's Awakening for the Game Boy:
Although they are not the exact same sprite (they are turned slightly to the side and are a bit longer), these skulls in the Yarna Desert area look strikingly similar to those unused skulls in A Link to the Past. It's almost as though someone was disappointed their giant skull graphic didn't get used in ALttP, so they put something similar here when they had the chance.