1. "Billions and billions." Carl Sagan never said this, and he even explained that he never said it in the first chapter of his book, which, incidentally, was titled Billions & Billions:
"Oh, I said there are maybe 100 billion galaxies and 10 billion trillion stars. It's hard to talk about the Cosmos without using big numbers...But I never said 'billions and billions.' For one thing, it's too imprecise."
The quote actually originated from Johnny Carson's impression of Sagan.
2. "The British are coming!" was very likely never shouted by Paul Revere. It was in the poem about his journey (Paul Revere's Ride), though. The thought is that most of the people in the colonies still considered themselves British, plus the whole mission was cloaked in secrecy - running through the village shouting may have thrown off the plans a bit.
3. "Let them eat cake!" has been attributed to the tragic Marie Antoinette for centuries, but she didn't say it. A similar quote was said by philosopher Jean-Jacques Rousseau in his autobiography Confessions: "I recalled the make-shift of a great princess who was told that the peasants had no bread and who replied: "Let them eat brioche." But he wasn't talking about Marie Antoinette. He was talking about an incident that happened 10 years before she was even born. The twisted quote was attributed to her probably to turn the country against her even more. Apparently, it worked.
4. "Houston, we have a problem." Not quite, not quite. When things went awry on Apollo 13, Fred Haise started with, "OK, Houston," and was then interrupted by Jim Lovell with, "I believe we've had a problem here," followed by "Houston, we've had a problem. We've had a main B bus undervolt."
5. "Can't we all just get along?" The context of Rodney King's famous quote is right, but what he actually said was, "People, I just want to say, you know, can we all get along?"
6. "I cannot tell a lie. It was I who chopped down the cherry tree." George Washington may have been honest, but he never said this statement. One of his many biographers, Parson Weems, made up the quote in the 1800s.
7. "Pride comes before a fall." Well, if you're quoting the Beatles, that's right. But if you're quoting the Bible, the saying is, "Pride goeth before destruction, and an haughty spirit before a fall."
8. According to WinstonChurchill.org, the quote "The only traditions of the Royal Navy are rum, sodomy and the lash," was never actually said by him. His assistant, Anthony Montague-Browne, said that Churchill wished it was his quote.
9. Mark Twain. Check out Snopes — it's full of misattributed Mark Twain quotes. They talked to the author of Nice Guys Finish Seventh, a book about Samuel Clemens. He said that any time a quote is anonymous, droll, and sarcastic, people pretty much automatically assume that Mark Twain said it. Here are just a couple that he never said (or they were misquoted):
"There are three kinds of lies: lies, damn lies, and statistics." He did quote this in his autobiography, but attributed it to Benjamin Disraeli.
"To cease smoking is the easiest thing I ever did. I ought to know because I've done it a thousand times."
"So I became a newspaperman. I hated to do it, but I couldn't find honest employment."
"Whenever I feel the urge to exercise, I lie down until it goes away."
10. "Ain't I a woman?" is supposedly a phrase Sojourner Truth delivered to much fanfare at the 1851 Women's Convention in Akron, Ohio. Although it's widely known as Truth's "Ain't I a Woman?" speech, she probably never said the phrase once, let alone the four times it's been said her speech contained it. So how did we get a transcript of an incorrect speech? We likely have feminist Frances Dana Barker Gage to thank. She was there the day the speech was given and published the version she remembered — 12 years after the fact. The version published the day after her speech was much different than the way Gage remembered it. If you check out both speeches, you'll notice that even the dialect is completely different.
11. Harry Truman may have popularized the saying "The Buck Stops Here," but he definitely didn't invent it. The sign that so famously resided on Truman's desk was actually a gift from Fred Canfil, a friend who saw the sign on the desk of a prison warden in Oklahoma. Canfil requested a sign for the President and it was mailed to him on October 2, 1945. Fun fact: the reverse side of the sign said "I'm from Missouri."
See also: 10 Famous Movie Misquotations.