"Con la Iglesia hemos topado" ("We stumbled upon the Church") is a popular misquote of Con la iglesia hemos dado, Sancho ("We found the church, Sancho" - small letter, as they are talking of a physical building) from Part II, Chapter IX. The stock phrase version is used in Spain to express annoyance at the meddling or lobbying of the Catholic Church (the institution) in a political matter.
"Ladran, luego cabalgamos" ("They bark, therefore we ride") or "Ladran, Sancho, señal que cabalgamos" ("They bark, Sancho, sign that we ride") is also atributed to the book but it actually comes from Goethe's 1808 poem Kläffer ("Barker"; obviously, without the interjection of "Sancho": :But their strident barking / is only a sign that we ride"). In its stock phrase version, it's used to say that an attack from one's enemies over a recently taken action is a sign that you are doing the right thing. There is an even more insulting version, "Ladran, señal que son perros" ("They bark, sign they are dogs").
"A donde fueres, haz lo que vieres" ("Wherever you go, do as you see") and its multiple variations. A lot of people think this idiom comes from the book, or rather that it's a variation of the quote "Cuando a Roma fueres, haz como vieres" ("When you go to Rome, do as you see"). However, although the quote does appear in the book, it's origin is actually from the 4th century, and it was first uttered in Latin by the bishop Ambrose of Milan.
Torch the Franchise and Run: As noted above, Cervantes was dismayed to see other writers producing unauthorized Don Quixote stories of their own, and wrote Part Two which finishes with Don Quixote regaining his sanity right before dying to give the character a definite ending.
Write What You Know: The tale of Ruy Pérez de Viedma, a Spanish soldier who was captured by the Turks and Made a Slave aboard one of their galleys, is probably based on the author's own experiences.