Mythbusters

Eating two Devil Fruit
Myth: When you eat 2 devil fruits, the second fruit will replace the first fruit's power

Fact: Eiichiro Oda stated in one of his SBS that you can't transfer the fruit's power to another person, if you take one bite of it the fruit becomes normal. This caused fans to think he meant if you eat the second devil fruit it will replace the first fruit but in Enies Lobby arc, Lucchi and Blueno said if you eat the second fruit after eating the first one, you'll explode.

Death Buster Call Star
Myth: Buster Call having laser beams

Fact: Fans just love to predicate Oda and hopes for the Buster Call to be an huge and scary Death-Star like ship. Packed with lasers that can blow up the island like how Death Star blows up Aldareen. Sadly, chapter 396 proves this wrong. The Buster Call turns out to be a military operation involving a fleet of World Government ships headed by five Vice Admirals, whose mission is to bombard the island until everything is burned down.

D Means Dragon
Myth: Dragon appeared in Logue Town and Luffy was able to hear the Dragon talking in the Apis Arc. The D must mean DRAGON.

Fact: The Apis arc (otherwise known as the Warship Island Arc) was not Canon. Therefore everything in it (except for a few things like the Calm Belt information) was not written in the manga by Oda. However it has been proven Dragon has a important plot role.

Pirate Summit
Myth: Ace said something about the pirate summit so it must be a place or event.

Fact: There is no pirate summit, what Ace meant was "See you at the top" but he did not mean that there will be a pirate summit. The "See you at the top" line was a figure of speech by Ace, challenging Luffy to become strong like him and join the top tier of strength. Pirate summit was an mistranslation found in many fan translations and sub translations. K-F, one of the Fansubbers mistranslated it, Stephen the script translator also mistranslated it as well and said he'll correct it soon. Also the piece of paper Ace gave Luffy was not an invitation, it was was never stated what was written on it.

Mihawk's and Shanks' Bounties
Myth: Mihawk carries a bounty of 600,000,000 and Shanks a bounty of 700,000,000.

Fact: If Oda gives a bounty, it will appear in the Manga itself either on a wanted poster or in a info box when the character is first introduced. Outside side of that, only the SBS will feature bounty amounts and estimates. There has yet to be a bounty (or former bounty in Mihawk's case) released for either character.

These rumoured bounty amounts were started off by fans, mostly put down to ones who create fan-based posters of the characters. There are many fans who believe them, some even insist the amounts are correct, however they are not. Confirmed bounties will also appear in catch up chapters like Grand Line Times.

Other bounties often seen are Whitebeard being worth 1 billion and Gold Roger being worth 1.2 billion.

The higher the bounty the stronger they are
Myth: Theres no way someone like (for example) Buggy can beat Luffy. He is worth 16,000,000 Luffy is worth 300,000,000. Buggy is far too weak.

Fact: While some characters in the series such as Bellamy used them as a power rating, they are not. Bounties are a threat level not a power level. Someone quite weak can have a high bounty and some quite strong can have a low bounty. This was made clear by Oda in one SBS. A good example of bounty level not reflecting power is Tony Tony Chopper who carries a mere 50 bounty.

Mihawk and Elements
Myth: Mihawk can cut all four elements, it says so in X Data Book.

Fact: This is an old forum joke let loose by translators of each book. There is nothing in the books about such a thing. However, this is one of those rumours amongst the fandom that doesn't just die easy.

Lucky Roux is fast
Myth: Lucky Roux is the fastest person in the Grand Line.

Fact: Yet another translator joke, simuliar in nature to Mihawk and cutting elements. The orginally rumour came once more from info in the Data books (which turned out to be a translator joke). This is based on Luffy's past when Roux appeared out of no where to kill a bandit. Again this is not actually a true fact.

Norland is a D.
Myth: His name is Montblanc D. Norland!!!

Fact: A joke that came directly from kaizoku-fansubs. Basically, they decided to do a stupidity test to see what would happen. Neverless their joke swept through much of the fandom and apparently it took several weeks to kill the myth, meanwhile it leaked into just about every forum (leding to many embarressments from people). His name is just Montblanc Norland.