Don’t look a gift horse in the mouth.
Another partial quote that is often misinterpreted to allude to a present changes when the entire quote is read. The meaning is clear that it has to do with manners and a tip.
‘Tis civility wins the gratuity, and those who expect them should take graciously what is given them and not look a gift-horse in the mouth unless they know for certain that the winners are common cheats and have won unfairly. Cervantes, Don Quixote