For this glitch, you need two files. For File 1, you need to have the Highwind and still not have Yuffie on your file. For File 2, you need to have saved just prior to the Motor Ball boss battle. Load File 1 and land near the Chocobo Ranch, preferably in a spot you can remember as it will be invisible during the glitch. Go through Mythril Cave as normal and save just outside the forest near Junon. Load File 2 and begin the boss battle. Have a Game Over in that battle. Load File 1 again and fight in the forest until you find Yuffie. Defeat her and, instead of the scene loading that usually does, you will be warped instead into the Motor Ball battle. Defeat it. (Note that this does not necessarily allow your File 1 characters to enter File 2; rather, it just alters a value in the game's data that says "when" you are in terms of plot, so there's no item duplication or the like.) Go to the Rocket Town side of Mt. Nibel via the Highwind. Land and make sure that Tifa, Cait Sith, Cid, and Vincent are not in your active party. Go into Mt. Nibel. It will take a moment for the game to let you control the character; wait for about 30 seconds until text appears on-screen from a flashback. Scroll through it, leave Mt. Nibel, and save in the overworld. Reload this file and kill two party members in one battle. After, use the PHS to swap them out for other characters, then kill the only other one from the original group and end the next battle. Swap all three KO'ed members into the party. Enter Mt. Nibel and then get into a battle, in turn getting a Game Over. Next, load the file on which you want to enter a Debug Room. Go to the Rocket Town side of Mt. Nibel, enter a battle in the overworld, and win. After, leave Mt. Nibel and go to Junon via the Highwind. In the overworld, enter battles in the grass at random and keep escaping battles until you find one with 2 Nerosuferoth and 1 Formula (25% chance to find). Escape this battle, too, then start flying and land in the Highwind immediately thereafter, and win the next random battle you find. You will then be warped into the Debug Room.