And finally, we come to a category of misquotation that has everything to do with our sensory perception: we misquote because we actually, physically misread or mishear (our eyes see what they want or expect to see; our ears hear what, in turn, sounds right to them)—and the result makes so much sense that it sticks.
In "The Mourning Bride," William Congreve makes a beautiful observation of music's appeal: "Music hath charms to soothe the savage beast." Actually, the word in question is breast. But see how close the two look? How easy it would be to read "beast" after "savage." The former almost cues the latter in mind. We think of savage beasts all the time. Savage breasts, not so much.
But perhaps the most frequent victim of this type of error is William Shakespeare, whose lines often look and sound like something else—and something that is frequently easier to recall later on than the original. In Hamlet, we often hear the prince described as "to the manor born." Only, when Hamlet refers to himself, it's as born to the manner. On its own, that makes little sense, a likely reason for the manor's persistence. But take it in full context—"But to my mind, though I am native here / And to the manner born, it is a custom / More honour'd in the breach than the observance"—and it becomes quite clear what the prince is trying to say.
And how often does one hear the evil cackling of the Macbeth witches, "Bubble bubble, toil and trouble"? In the play, of course, the proper line is "Double, double"—but the two sound remarkably similar (especially with a British accent), and we're talking about a cauldron here, right? So bubbling makes a great deal of sense.
There is, too, that well-known line from the Merchant of Venice, that admonition that "all that glitters is not gold." (Sometimes, it's also quoted as "also that glistens.") In reality, nothing is glittering or glistening. What Shakespeare wrote was, "All that glisters is not gold." But it's easy to see how the misquotation might arise—and the replacements certainly sound better to the modern ear, and make more sense.
That's the thing about misquotations. They tend, for the most part, to arise not out of malice or intentional misrepresentation but out of understandable cognitive processes. (That, and improper punctuation. Remember Jessica Dovey, the inadvertent heir to Martin Luther King?) And the more understandable a process, the more likely it is to play out in similar fashion for multiple people—and the more likely the misquotation is to spring up at various times and in various places, instead of being immediately corrected.
Of course, the other common reason for misquoting is simple laziness. We think we remember something and so we just write it down, rather than spend time checking. Or, we like the way a phrase sounds or the message it has and so we just assume our (likely online) source is correct—and the more sites there are with the mistake, the more persuasive it becomes—instead of painfully tracking down the original to verify it for ourselves.