Misquotation and missed attributions happen all the time. ALL the time. Even people you think would get it right--say, JFK, who was a prolific re-quoter of others and had a speech-writing staff to boot--often got the attributions wrong.
But misquoting happens for some good reasons.
The primary reason is that when using quotes, the reference we're most likely to consult is our memory. This is a hazardous form of research. Our memory wants quotations to be better than they usuall were, and said by the person we want to have said them. (p xi)
It's an excellent point that's supported by all the cognitive science memory research. Memory is fickle and errorful in specific ways. We tend to remember the most common version of a story, rather than what actually happened, and we tend to remember things in comfortable ways, with excess baggage trimmed for recollection.
When it comes to quotations, memory is too much the servant of aspirations, not enough the apostle of accuracy. That is why misremembered quotations so often improve on real ones. Memory may be a terrible librarian, but it's a great editor. Excess words are pruned in recollection and better ones added. The essence of a good remark is preserved, but its cadence is improved. Churchill's "blood, toil, tears, and sweat" becomes "blood, sweat, and tears." Durocher's "The nice guys are all over there. In seventh place" morphs into "Nice guys finish last." Gordon Gekko's "Greed, for lack of a better word, is good" ends up as "Greed is good." (p xi)
In essence, quotations get improved over time with retelling, the rough edges smoothed and the cadence made right.
They ALSO shift location from who might have really said something to someone who might-have-plausibly-said-something.
The misattribution process is not random. Patterns can be discerned. If a comment is saintly, it must have been made by Ghandi (or Mother Teresa). If it's about honesty, Lincoln most likely said it (or Washington), about fame, Andy Warhol... about courage, John Kennedy...about winnings, Vince Lombardi... malaprops by Yogi Berra. If witty, a quip must have been Twain's concoction, or Wilde's, or Shaw's, or Dorothy Parker's. (p xii)
We remember things because they fit into a larger pattern, that is, they make sense in a particular context and don't fit in so well to some other.
A quotation often attributed to Nelson Mandela takes this form: "Our deepest fear is not that we are inadequate. Our deepest fear is that we are powerful beyond measure. Is it our Light, not our Darkness that most frightens us." When any source is given at all, this is said to be from an inaugural speech by South Africa's two-term president. Aside from the fact that these words don't even sound like him, they do not appear in either inaugural address given by Mandela. On the other hand, those sentences can be found in the 1992 book, A Return to Love by pop theologian Marianne Williamson. (p xiv)
The elevating and uplifting tone of the quote makes more of an impact on the listener/reader if it comes with the moral authority of Mandela. It's less effective (and less memorable) if it's from Williamson. And so we tend to remember what makes sense together, rather than what actually is.
As a consequence, we remember the common and what we want to be true. And in an age of fast and facile internet search for quotation sources, it's easy to be susceptible to confirmation bias--you find something you expect to be true, something that's easily relinking, copied and pasted or retweeted, and you think it's true.
But more on this in another post. In the meantime, remember that just because something has been repeated a million times on the web doesn't make it true. It just makes it repeated, and we all know how easy that is to do.