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Cervantes had spent years promising the second part of Don Quixote when, in 1614, an impostor beat him to it. Someone writing under the name Alonso Fernández de Avellaneda — his true identity remains unknown — published his own sequel, complete with insults to the author in the prologue.
Cervantes struck back from inside the novel itself. In his genuine second part, published in 1615, Don Quixote learns that a false book about him is circulating and, to prove it wrong, changes his plans: instead of riding to Zaragoza, as the impostor's version had it, he heads for Barcelona.
There's more: Cervantes borrows a character from the fake book, Álvaro Tarfe, and has him swear before a notary that the true Don Quixote is his.
Four centuries later, the fake Quixote is remembered for one thing only: the reply it provoked.