As some long-term readers know, I have an interest in philosophical folklore; much of this folklore is found in discussions of critical thinking and informal logic, which is a pretty fruitful hunting ground for it: almost everything that goes by the name 'critical thinking', and much in the field of informal logic, is philosophical folklore, bits and pieces that have filtered down and become common legend. Some of these bits and pieces, used wisely, do good work; others are, as we might say, mere superstition -- things that were once held by only a few for very specific and very controvertible reasons, or things that have long since become too crudely simplified to do the work they are supposed to do, or things that once made good sense but are now consistently understood a completely different way due to the accidents of linguistic change.
But one finds philosophical folklore elsewhere, especially when it comes to history of philosophy. I've mentioned, for instance, the irony that what is often called Leibniz's Law is not found in Leibniz; you can find superficially similar claims, but on closer claim they turn out not to have anything to do with identity in the proper sense, or else to be obviously different in logical character from what goes by the name. For instance, Leibniz explicitly tells us that his claim in Discourse on Metaphysics, section 9, which is often said to be the source, is paradoxical, in the company of claims like "You can't divide substances in half" or "Every substance mirrors the entire world." It's also based purely on Leibniz's very peculiar account of what an individual substance is. Nothing like this is even remotely in view when people talk about "Leibniz's Law". In this case what seems to have happened is that some claims made by Leibniz were put into a very different logical form than Leibniz himself would or could have put them, and by people making assumptions Leibniz himself wouldn't or couldn't have made, with the result that things were changed significantly in the translation.
Another example is that what is usually called Pascal's Wager actually does not derive directly from Pascal, although it was influenced by him. It really derives from Arnauld and Nicole's Port-Royal Logic and you can recognize it by the fact that it gives a role in the Wager to hell -- i.e., very bad consequences play an important role in the argument. This is not the way Pascal sets up his own Wager, in any of the fragments we have. What happened there is that Port-Royal Logic was published long before anything directly from Pascal (we only have Pascal's version in fragmentary notes published posthumously), and therefore it had a chance to become very widespread, although, since it clearly is influenced by Pascal, it gets Pascal's name.
Another example, about which I've been meaning to write a post (but it's a complicated issue and so I need to have a good stretch of time to do it), has to do with the phrase "Knowledge is Power". It's usually attributed to Francis Bacon. It is indeed a very Baconian sentiment, and Bacon does have the Latin phrase scientia est potentia in his writings. But there he's talking about divine simplicity, and the claim he is making is that knowledge is (the same as) power for God, as opposed to us. One occasionally also finds it attributed to Thomas Hobbes, and indeed, Thomas Hobbes also says that knowledge is power; and the meaning is much more like what we usually mean by it. But he adds sed parva, roughly, "but only a little", and that's a pretty important qualification.
The list could be extended at considerable length. Not all bits of philosophical folklore are wrong, it should be said, although explaining the ones that are wrong is often a more interesting task than explaining the ones that are right. And even those that are wrong if taken straight may nonetheless show some real insight -- "knowledge is power" is indeed a pretty good summary of Baconian philosophy, even if Bacon never thought to summarize it exactly that way, and it would be virtually impossible to come up with a three-word summary that does better -- or be based on conflations or errors that even very reasonable people may make.
What sets me in mind of all this is this discussion of a bit of philosophical folklore, by T. H. Irwin. One occasionally encounters the claim that Augustine argued that the virtues of pagans were merely 'splendid vices', splendida peccata. As with most such philosophical folklore, one can make some Augustinian sense of the claim (although, as Irwin notes in the paper, how much Augustinian sense is a tricky matter to determine, with lots of room for controversy); but Augustine himself seems never actually to say this. This is a particularly interesting case because it has been a massively influential bit of philosophical folklore. Because philosophical folklore is popular and widely accepted, it does tend to have some real influence, which is why it's very worthwhile for historians of philosophy like myself to study it (and a real shame that we don't do it more often). But the degree of influence of this particular tidbit through history has been quite extraordinary; there are probably only a relatively small handful of folkloric tidbits (like Ockham's Razor) that have been more influential.