This video features about half an hour of a guy doing nothing but walking up and down stairs in Final Fantasy for the NES. Don't say I didn't warn you.
Speedrunner gyre_, who set a new "any%" world record on Sunday by "completing" Final Fantasy in 59 minutes and 10 seconds, used a save-corrupting glitch to make his game think that A) he'd already gotten all four orbs of light; and B) his characters were over level 99, therefore leading enemies to run away. To do this, he entered specific commands as his character names and then overloaded the game's memory—by walking up and down stairs—until wonky things started happening.
"This glitch has been known about for a while," he says, "but people didn't understand how to take advantage of it until quite recently."
You can watch the whole video here (skip to 27:38 if you don't want to watch stairs):
So what's with all the stairs? Allow gyre_ to explain:
Your party names are stored in some scratch memory when you enter them on the character select screen. As long as you soft reset, it'll still be there when you start up another game. What I did was a stack smash attack where by walking up and down the stairs 70 times it would start executing code nearby that scratched memory. I had to pick a couple things correctly, like my character classes and so on, for it to actually reach my character names, but it'll execute my second character's name.
When you saw me enter two codes what I was actually doing was jumping from the second character to the fourth character to get a couple more bytes of space, since I only have four bytes of executable code in these character names. You really can't do much, you're extremely limited in CPU instructions cause there's only a handful of characters. And then there's all these instructions, which take two or three or four bytes to encode. So that becomes an issue—how do you actually input those in the game. A lot of it's trying to chain to other parts of memory where you actually can do useful things by taking advantage of code that's sitting around.
In other words: magic.