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 tranfer 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 frist one, you'll explode.

Death Buster Call Star
Myth: Buster Call having laser beams

Fact: Fans just loves 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, in 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 they'll be an 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 tiar 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.