How OP was it? Thin Mushroom (rapid-fire) + Spoon Bender (homing tears) + Mom's Contact (freezing shots) + Parasite (tears split into two when they hit an enemy). What happens is that the enemy is under a constant three-way barrage of tears: One stream coming from Isaac himself, the other two streams splitting off because of Parasite, and then looping back onto the enemy (because of Spoon Bender). And since the freezing effect of Mom's Contact is chance-to-happen-per-shot, the rapid fire makes the freezing effect pretty much infinite.
The very fact that he made so many videos of The Binding of Isaac is one. Further proof: he finished a Wrath Of The Lamb playthrough on his second try.
He got the Mama's Boy achievement without even realizing it until it popped. He actually got hit during Womb 1—however, he finds a Forget-Me-Now in the deal with the devil room, and for the hell of it, takes it and redoes the Womb 1. He proceeds to not get hit for the Womb 1 and 2, resulting in the achievement.
Beating the desert level of Gods Will be Watching on his second try, albeit by a ridiculous stroke of luck.
On Episode 20 of his playthrough of the XCOM: Enemy Unknown mod, Long War, he fights his first Terror mission, accepting the fact that his entire team will likely die right out of the gate. Sure enough after the first turn, four Chryssalids charge up to his team, four Floaters come in from the other side, and three more show up to attack him from the front leaving his entire team flanked by at least one enemy. Not only does he not lose his entire team, he doesn't lose a single soldier (though one was critically wounded) and he manages to save 14 out of the 18 civilians.