Mythbusters

Eating two Devil Fruit
Myth: When you eat 2 devil fruits, the second fruit will replace the frist 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 frist 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. 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 is nothing more but an huge simple wooden ship that fire simple cannon packed with large explosion and manage to set the whole island on fire.

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.