In his Amnesia: The Dark Descent playthrough, at the beginning of the game, he manages to throw a chair up at a chandelier and have it HOOK onto the thing...And he does it on his first throw, WHILE the chandelier is swinging...Yeah.
Another one from Amnesia: The monsters in the game make most people run and hide. Does Over The Gun? Nope. He waddles over for a closer look at the thing. Balls of steel.
In the Half-Life 2 playthrough, he gets stuck on a portion of the Ravenholm level (a zombie-infested town if you don't know), where you have to pull a lever to move a platform so you can jump up onto a rooftop. He proceeds to ignore the lever several times, despite it being in plain sight. Then, after being frustrated for about 15 minutes, he stacks up a bunch of barrels and hops up onto the rooftop. That's right, he fails to see a lever despite looking straight at it, but then solves the puzzle Gmod-style: stack shit up and jump on it.
In his HTSF of Super Metroid he enters the Wrecked Ship backwards through Maridia
There's another awesome moment way back in his first IWBTG HTSF where, after getting several complains from people that he wasn't as good as he claimed he could be, he switched on Gear 2 and did a runthrough of the route to Kraidgeif without dying once. If you'd like to know how hard it is, look up some other videos on the route. If anything, the video shows what Over The Gun is capable of when he's serious.
Another awesome moment on the topic of boss fights is OTG deducing the way to beat Vamp, without assistance from the chat. To be more specific, the fact that you have to use the syringe to suppress the regenerative abilities of his nanomachines, thus allowing you to actually kill him. It's a small moment, but a notable one, given that the title of his videos is 'How To Successfully Fail', after all.