In the "Author's Preface" to Part 2 of Don Quixote, we learn that Miguel de Cervantes has a major beef to settle. It turns out that some jerk went out and published a sequel to Part 1 of Don Quixote before Cervantes had a chance to write his own.
Cervantes more or less tells us that this knock-off Don Quixote is the reason he chose to kill off the title character at the end of his own version. He writes that in his version, "you have the Knight once more fitted out, and at last brought to his death, and fairly laid in his grave; that nobody may presume to raise any more stories of him" (2.1.AP.8). He repeats this same point at the close of the novel, pleading with people to leave his character alone by writing, "[L]et the wearied, mouldering bones of Don Quixote rest quiet in the earth that covers them" (126.96.36.199).
What is even more amazing, though, is that Cervantes actually writes the false sequel into the world of his own sequel, having Sancho and the Don come across this horrible book during one of their adventures. Don Quixote wants to go the Spanish city of Zaragoza, but after finding out that this is what happens in the knock-off version of his adventures, he vows: "For that reason […] I will not set a foot in Zaragoza, and so the world will see what a notorious lie this new historian is guilty of" (188.8.131.52).
In other words, the fictional character of Don Quixote vows never to go to the city of Zaragoza in order to spite a real-world author. Now that's what they call some serious metafiction. It's also a cool variation on one of the novel's main themes: the way fiction influences real life. Here we've got fiction (Don Quixote's books) influencing real life (Don Quixote's quest) influencing fiction in the real world (the knock-off sequel) influencing a real author (Cervantes) and fictional characters (Don Quixote) and much of the course of a fictional novel (Part II of Don Quixote).
Just try to picture that all at once. We dare you.