The 5-4 skip in Yoshi's Island is still one of the most impressives feats I ever saw in a classic game.
In this level, you need to get a ride from a platform monster in order to cross a large room. But 2stop, a Yoshi's Island speedrunner, found a way to skip the whole ride by floating through the bottom of the room, while still getting all itens.
If I remember well, this was found back in 2013. It starts at 1:59.