OMG WTF? OR most irritating/awful/annoying book ending
Easy. The original final chapter of A Cl0ckwork Orange.
Needless to say, there are spoilers here. If you don’t want to hear about the ending to the book (and film), STOP READING NOW.
So, as originally written, our protagonist Alex goes on his horrible rampage, is brainwashed to be a good person, is basically destroyed by that, and is turned back into a little bastard of a thug. Then, in the last chapter, a far older and wiser Alex reveals that he basically grew out of his horribleness, and that humanity is good, blah blah blah.
It totally ruined the point of the damn book.
Fortunately, the American publisher realized this, told the author he was an idiot, and cut it, so that it ended with Alex being a horrible little bastard again. It’s a far more effective ending, and actually fits with the rest of the book. I’ve no idea why Anthony Burgess thought his sweetness-and-light ending was a good idea, but he was wrong.