"This planet is very old," he continued. "Like the works of Homer and Hesiod, who can say through how many editions it has passed in the immensity of ages? The rent continents, the straits, the gulfs, the islands, the shallows of the ocean, are but vast fragments on which, as on the planks of some wrecked vessel, the men of former generations who escaped these commotions, have produced new populations. Time, so precious to us, the creatures of a moment, is nothing to nature. Who can tell us when the earth will again experience these fatal catastrophes, to which, it appears to me, to be as much exposed in its annual revolutions, as are the vessels which cross the seas to be dashed in pieces on a sunken rock? The near approach or contact of one of those globes whose elliptical and mysterious courses are perhaps the agents of our destinies, some variation in its annual or diurnal rotation, in the inclination of its axis or the equilibrium of the seas, might change its climate, and render it long uninhabitable."
For more than a century following the publication of this passage in 1801, historians cited it as fact, including in James Parton's famous Life and Times of Benjamin Franklin (1864). The only trouble is, Franklin never said it.
The French author, Guillaume Jean de Crèvecoeur, was a serial plagiarist who invented not just Franklin's speech, cobbled together from other authors' work, but Franklin's entire trip to Lancaster. He was actually at the Constitutional Convention in Philadelphia. This incident became yet another in the parade of pseudohistory and false fact masquerading as truth. Sadly, most scholars simply accepted the story as true, and many took elaborate steps to justify their belief when difficulties--like Franklin's presence in Philadelphia and absence from Lancaster--made belief difficult.
It took until 1947 for someone (in this case Perry G. Adams) to do the research needed to conclusively debunk the lie and the mountains of post hoc justifications used to prop it up. How did he do it? He read the sources Crèvecoeur plagiarized and showed how they were stolen, word-for-word and altered to fabricate Franklin's speech. The clincher? The source, a letter by a Jonathan Heart, wasn't written until 1791 or published until 1797. Franklin couldn't have copied what didn't exist prior to his death, nor could Heart have copied Franklin since the "speech," supposedly a private conversation, was only published in 1801. The only solution: Crèvecoeur faked the whole thing. While Heart's letter and Franklin's speech about American history were virtually identical, there was one difference: the passage about the comet. That, apparently, was Crèvecoeur's own unique take on ancient history.