Reprogramming Super Mario World
This guy beats Super Mario World in 6 minutes. He uses the credits warp glitch to reprogram the game while playing it.
The exploit works by transferring control to sprite X-position table via the chuck eating glitch so that the game executes the X-position of sprites as if it were code. Using koopa shells, the X-position table can be filled with values which switches the game from mode 0x14 (normal level execution) to 0x1C (midway through the credits).https://www.youtube.com/watch?v=14wqBA5Q1yc#t=24
EDIT: Today Carlsagan42 finished the game under 2 minutes.