Terrence in KateModern — goes from smug, quietly intimidating badass in series 1 to raving, giggling psychopath by the end of "Precious Blood".
The Shadow and Rupert van Helden have also suffered this by The Last Work.
In Doctor Horrible's Sing-Along Blog, the main character is shown as a Technical Pacifist and Ineffectual Sympathetic Villain who states that killing isn't his style. However once his Jerk Jock archnemesis Captain Hammer states that he's going to steal the girl of his dreams just to torture him, he then goes into a Villain Song where he swears to kill him. This is followed in rapid succession by Captain Hammer's own breakdown when he is made to feel pain for the first time in his life (it's hard to call it a HeroicBSOD when he's shown to be just as evil as the protagonist), and a second, deeper one from Dr. Horrible when his victory coincides with the death of his Love Interest.
In Lost Boys Of The Cascades, Barry suffers a villainous breakdown after his attempt to frame the Klamath Indians for murdering Jennie, Troy, and Dylan falls apart due to Troy surviving and telling the truth.
In The Spoony Experiment review of Street Fighter 2010, Doctor Insano suffers a breakdown after failing to create the ultimate being (instead producing a cute pink blob thing). He begins crying about how his science never works and nobody loves him before his new "son" cheers him up.
No... No... No, no, no, no, NO! MOTHERFUCKER!!! NO!! NO! NO! NO! BULLSHIT!! BULLSHIT!! BULLSHIT!! Okay, okay, okay. Calm down, Dio. Calm down. Calm down.
The Evil Overlord List refers to this a few times, most famously #24: "I will maintain a realistic assessment of my strengths and weaknesses. Even though this takes some of the fun out of the job, at least I will never utter the line "No, This Cannot Be! I AM INVINCIBLE!!!" (After that, death is usually instantaneous.)"
"You... I have to admit, you're not the first guy on this island to seriously piss me off, but if there was a competition going on you'd of just won first fucking prize."
Freeza in Dragon Ball Abridged goes through this a few times in season 2. Most notably in episode 25.
"Oh, I'm sorry. I'm usually far more composed. I'm just a little bit ABSOLUTELY LIVID."
He becomes even more so with Goku, not because of his resilience, but his absolute stupidity gets on his nerves.
