According to Wikipedia (not the most transparent of sources, but hear me out):
The most oft-cited Voltaire quotation is apocryphal. He is incorrectly credited with writing, “I disapprove of what you say, but I will defend to the death your right to say it.” These were not his words, but rather those of Evelyn Beatrice Hall, written under the pseudonym S. G. Tallentyre in her 1906 biographical book The Friends of Voltaire. Hall intended to summarize in her own words Voltaire’s attitude towards Claude Adrien Helvétius and his controversial book De l’esprit, but her first-person expression was mistaken for an actual quotation from Voltaire.
Good. Now can we please retire that “I will defend to the death your right to say it” quote from liberal discourse? It is the grown-up version of that saccharine Camus quote of “Walk beside me and be my friend,” in that it was a good idea at first but it’s been parroted out so many times it’s lost the emotional impact it once had. The Camus quote is the refuge of 12-year-old girls who don’t know who first came up with it; the pseudo-Voltaire quote is the much-abused plaything of adults who ought to be able to come up with their own slogans.
(It popped up, predictably, at the Rally to Restore Sanity on Saturday. Most of my fellow attendees wrote their own material.)