Flawless run of the Impossible Boy achievement in Super Meat Boy
SPOILER: once you beat the Light World in Super Meat Boy, you unlock Cotton Alley: a bonus world that lets you play as Bandage Girl, listening to bubbly music while sawblades slice you up again and again. Get an A+ in all of those levels, and you can play through them in the Dark World, where they're exponentially more difficult. Beat all the stages without dying, and you'll unlock the Impossible Boy achievement, which a measly 0.1% of SMB players have accomplished. Record yourself doing this in one clean sweep and post it to YouTube, and you become a legend. SPOILER AGAIN: watching this video may bring you to enlightenment.
I don't know about you, but if I had reached the second-to-last level without dying yet, my palms would be sweatier than wearing mittens in a sauna. Kudos to Bullexcrements aka srakaaaaaa for literally achieving the Impossible, with some heart-stopping close calls thrown in for good measure. Check out the rest of his channel for more SMB nuttiness, including a valiant effort by Tofu Boy and a Wood Boy run with quite the varied cast.