FF7 - Disc Swap Glitch (1998 PC version only)
One thing FF7 PC is known for within the community is how glitchy and poorly optimised it is for the platform. Over the years, some pretty insane glitches have been discovered and my small contribution builds on the findings of SuperAcro: essentially, if the disc is swapped during a battle swirl the game will run a debug routine and load a fight called 'Encount Error'. Winning/Fleeing this fight teleports your party back to their last known World Map location.
However, you can take this further by converting the Encount Error battle into any enemy formation in the game by first finding and then dying to that enemy formation, loading another save (without closing FF7 down), and then performing the Disc Swap glitch. This loads the encounter you died to, rather than the Encount Error one.
The uses were fairly limited, and useless for a speedrun as it falls under hardware manipulation. But I got some fun out of it, and we combined it with Yuffie Warping to jump to the end of the game and then finish it without having to fight any of the final bosses. Unless you count Yuffie as a final boss. Which she is.