Everybody loves countdowns, right?  Right.  So, I’ve come up with my own list of things people get wrong about Jefferson, based on my extensive observation of the stuff people put on the Internet or ask us about.  Here goes:

1. Thomas Jefferson wrote the Constitution. That would be the Declaration of Independence. 
2. Thomas Jefferson invented coathangers/triple-sashed windows/skylights/polygraphs/dirt. Well, this is somewhat debatable, depending on what your definition of an “invention” is, but the party line now is that Jefferson invented only the moldboard plow (and possibly the Wheel Cipher) but not any of those other things.  However, I will say that Monticello uses a pretty strict definition of "invention," and I'm told by people who know about such things that Jefferson could well receive patents under current rules for things that we would only say he "contrived" or altered from others' designs.
3. Thomas Jefferson said [x]. TJ said lots of things, but strangely enough, not most of the things people think he said. We keep a list of the most repeated Spurious Quotes attributed to Jefferson..
4. Thomas Jefferson was a Democrat/Republican/liberal/conservative. Everybody wants to claim Jefferson for their political point of view, understandably enough.  However, labeling Jefferson as strictly anything is a bit dicey, seeing as how a lot of time has passed and the world has changed quite a bit.
5. Thomas Jefferson grew marijuana. This is a very popular idea, and I even saw it on TV recently.  Jefferson did grow hemp, but as I understand it, the type of industrial hemp grown to make rope and cloth and so forth contains only minimal amounts of the compound that makes so many people so happy. 
6. Thomas Jefferson was an atheist/Deist/not a Christian.  As far as I know there is no good argument for calling Jefferson an atheist. I've heard some good arguments for calling him a Deist, and although he seems never to have explicitly called himself a Deist, he did hold some Deisty sort of beliefs.  The Christian/not-Christian thing is tricky; Jefferson's non-belief in the divinity of Jesus disqualifies him according to many folks' definition of Christianity.  On the other hand, TJ seemed to consider himself a Christian, in the sense that he was a follower of Jesus.  So let's just say, this is complicated, and quite often folks get tripped up with this because they oversimplify. 
7. Thomas Jefferson was my ancestor/relative. Well, that could be, actually.  Statistically speaking, however, most people would be wrong about this.  Perhaps surprisingly, it seems that the majority of people with the last name Jefferson are not in fact related to President Jefferson (see the famous study at the University of Leicester, which found that of 85 males tested whose last name was Jefferson, only two carried the same rare Y chromosome type as TJ.)
8. Jefferson was the first to bring vanilla/macaroni & cheese/ice cream to the United States. Jefferson was most likely not the first person to bring any of these foods to America, although his name probably has become attached to them because these were foods he served at dinners during his Presidency, which tended to be highly remarked-upon. So perhaps one could say that although he didn’t introduce them, he may have played a large part in popularizing them.
9. Jefferson was a Freemason. Negatory.
10. Thomas Jefferson bought a Qur’an so he could study his enemies and fight the Barbary pirates. Jefferson purchased his Qur'an in 1765, so it's unlikely he was thinking about dealing with the Barbary pirates at that point.

So there you go.  I apologize if your favorite Jefferson misconception didn’t make it onto the list, but you can always post it in the comments.  I love misconceptions, as long as they’re not mine!