Tower to Tower. It was a trick proposed for Halo 1 in 2004, and finally completed in 2011. The goal is to launch from the top of one blue beam tower on the level Halo to the top of the other. Those two locations are extremely far apart, extremely small, and neither were supposed to be accessible to the player in the first place. It's basically completely insane. The best horizontal launching techniques that were developed were just barely (mere frames on a ~7-second launch) fast enough to not result in a fall timer death, and you have to precisely strike an angled surface just right so the regular fall damage doesn't kill you. Many of the tricking techniques involved in the trick's final successful setup simply didn't exist when the trick was proposed in 2004, and even within months of its completion in 2011, people were still debating whether it was actually possible.
Full video, including history and setup [https://www.youtube.com/watch?v=hNnIG_t-TE0]
The launch by itself [https://www.youtube.com/watch?v=ZnacGaLlnps]
//========================
Hehe, that TwitchPlaysPokemon thing is amazing.