In the Donkey Kong Country 2 LP, during his run of the level Klobber Karnage, due to the emulator he was using (ZSNES) caused an emulation glitch that made the barrels spin without stopping, when normally you could control those ones with no problem, which made everything 10x harder than it really is... and he beat the unwinnable level, with some help, of course.
In his Ocarina of Time LP, he mentions having done the WaterTemple without using the Zora Tunic the first time he played. While this is possible, the Temple is much easier with the Zora Tunic.
He has beaten Contrawith one life. Not to mention, he plays the game like he does it every morning. What makes it more awesome? The next time he plans to beat the game, his special guest? His Mom. Manly.
Also some of his Self Imposed Challenges can go here. Did a Three-Heart Run of Zelda 2, beaten Super Ghouls n Ghosts, and did Legend of Zelda 1's Second Quest without the sword. He is crazy.
The Super Mario Galaxy 2 Part 47 video under Ensemble Darkhorse was so much so that it was the first of NCS's videos to reach 1 million views.
In his LMAO of Mario Party 2, he ends up dominating Mystery Land with a whopping 8 Stars, 4 times the amount of all the other players combined (Wario and Luigi had ONE Star each, while Donkey Kong got none), and swept all the Bonus Stars.
He also got 175 coins in Space Land after beating Luigi in a dual minigame worth 83 coins.(Bankrupting Luigi in the process.)
In his Link to the Past race, he won by a landslide, beating the game in 2 hours, 12 minutes and 37 seconds, and beating his old record in doing so.
And he would have done even better had he not died against the Helmarock King, his only death in the whole project (and he was also the last one to die).
NCS taking a stand against some of the more vocal haters near the end of his Donkey Kong 64 LP.
He manages to beat Final Boss Steven despite failing to heal his Pokemon beforehand (due to not knowing it was coming).
After struggling the whole game with it, him beating the entire final world of Yoshi's Island with 100% on his first run-through (including the final level, which means not taking a single hit from any variation of the final boss.)
He did a Mega Man 1-5 marathon stream on Christmas Eve, 2013. By the end of Mega Man 5 he was exhausted, out of energy tanks and clearly frustrated after several failed attempts at the final boss (especially since he didn't have Beat). However, he eventually won after being taken down to 2 bits of health and said that giving up wasn't an option because he couldn't let his fans down.