Batman's rogues gallery is home to all sorts of supervillains who many would probably classify as being among the criminally insane. They're dropped off at Arkham Asylum once they take their licks from the Dark Knight - which is a medical facility first and a super prison second - and most clearly aren't healthy.
The unhealthiest of all is clearly the Joker, a man who revels in violence, unpredictability and cruelty to seemingly no end. He's unbreakable, an amoral figure with an unconscionable attitude who can never, ever break character. He trolls Batman for a living, and no matter how much of a beating he ends up taking, you can be sure he'll still be smiling that same old smile by the time he's thrown back in his cell.
But that's not always the case. Although the Joker has developed a reputation for being invincible in the sense that he's impossible to break, a few characters have actually achieved that feat.
Whether by proving him wrong, finding an insult that lands or simply through scaring him half to death, the Clown Prince has been reduced to tears, rage or even quiet resignation by even the most unassuming of his adversaries. It's rare, but it does happen - and as soon as it does, you know something isn't right.