Posted some time ago in a cult...
1. The claim that the canonical gospels are not the earliest gospels. Claims that suppressed Gnostic gospels are the earliest written gospels and the canonical gospels were selected from among 80 other gospels. Firstly, there were only less than half of that many books written about Jesus' life. Second, the two Gnostic gospels Brown relies on weren't written until the 2nd c. A.D., after the New Testament.
2. Claim that the Dead Sea Scrolls were discovered in the 1950’s. No, they were discovered in 1947.
3. Claim that the Dead Sea Scrolls and the Gnostic texts found at Nag Hammadi are the earliest Christian Records. In fact the dead sea scrolls are JEWISH documents that don't mention Jesus.
4. Claim that Jesus Christ never claimed to be divine and was never worshipped as a deity until the Council of Nicea in 325 A.D. He's called "God" and "Lord" in the new testament several times, which is before 325...
6. Claim that Jesus was married to Mary Magdalene. Brown uses a quote from a Gnostic gospel that says Jesus was the "companion" of Mary Magdalene, which he says really means spouse because that's what the word means in Aramaic. Slight problem. the document he's citing was written in Coptic. Not Aramaic.
7. Brown calls it "Fact" that a group called the "Priory of Sion" exists and there are documents to support it in the National Library. Unfortunately these documents were long ago recognized as forgeries put in place by an anti-semitic supporter of the Vichy regime.
8. Claim that John in the Last Supper is Mary Magdalene. In fact his affeminate depiction was typical of the period (ask an art historian). The "omg it looks like an M!" thing is just plain silly.
9. Claim that the Merovingians founded Paris... uh... What? No. No they didn't. Thanks for coming out, Dan.
10. All those claims about Cathedrals, masterminded by the Knights Templar, representing parts of the female body. Don't even know where to start here... first of all, the description he offers doesn't really have anything to do with actual Gothic architecture. Usually Gothic churches have 3 main entrances on the west side, plus three more on the north and south. Explain that one in anatomical terms, please. Also the long rectangular nave was a translation of earlier Romanesque styles. ALSO, the Templars had nothing to with cathedrals, which were commissioned by Bishops.
11. Claim that "Jehovah" is a combination of "Jah", the masculine, and "Havah", the pre-Hebraic name for eve. Except that's wrong, again; "Jehovah" is a 16th century rendering of "Yahweh" using the vowels of Adonai.
12. Claims about Da Vinci. These are the worst, to me. The idea that the Mona Lisa is a self-portrait... what is he thinking, it's widely known that it's of a real woman, Lisa, the wife of an Italian named Giocondo. He says that the lack of grail in the "Last Supper" is meant to show that the grail isn't material, but the painting is about betrayal and not the Eucharist, and is based on St John's gospel. The institution narrative is in the gospel of Luke. His confusions about what actually goes on IN the painting are result of another error: he says it's a fresco. If it were, we'd have a clearer picture of what's going on in it, but it isn't. It's this weird idea Da Vinci had that didn't work out quite so well called a "Tempera", which led to whole pieces of paint to fall off the wall. The painting we have now is the result of a 2-decade-long attempt to re-create the original, and there's a lot of debate about whether the restorers "got it wrong".
Finally, the "Disembodied hand with the dagger". No, sorry. First of all, it's a knife, not a dagger. A dagger would have a thin blade with a sharp end for stabbing. This has a single edge and is clearly for cutting food. Secondly, the arm isn't "disembodied"; there are 6 disciples to Jesus' right in the painting, and a total of 12 arms and 12 hands. The knife is in the right hand of Peter. There's an entire book on this called the "Study for the Right Arm of Peter" in a private collection in England. Brown is just plain wrong.
There are dozens of others.
http://www.irr.org/da-vinci-code.html 
