Well basically the idea of this thread is to find any things that don't make sense or couldn't actually happen in the One Piece world and to try and give an explanation to solve it (lame explanations such as its a shonen manga aren't allowed :P)
Ussop in general - how is he still alive….
He gets completely battered in every single fight he's in yet manages to walk away unhurt. He gets hit by a 4 tonne bat to the face in alabasta then a couple of minutes later he's running around town looking for the bomb. Jumps off a 100ft tower as its blown up yet only cracks a few ribs, and can still run about the place. He must have broken his nose at least 10 times within the space of a couple of months.
A possible explanation could be that he's eaten a devil fruit that helps him heal quickly. This could be true for two reasons. Firstly whenever Usopp is seen in water, White sea in skypiea, crocodile's casino, Enies Lobby he has needed someones help to get out. Secondly out of all the Straw Hats he is the one who gets beat up the most yet he is never injured for more than a couple of hours.
Sanji's Devil kick - how does it work....
He spins so his leg catches fire....why? How does setting your foot on fire make it more powerful anyway apart from the fact that you burn them as well as kick them. Why doesn't he hurt himself?
A possible scientific reason for his leg catching fire could be to do with centrifugal force and friction of the air and stuff. Or alternatively the spinning could be used to confuse his opponents while his hand sneakily reaches into his pocket for his cigarette lighter. As for why it doesn't hurt him, the possible reason could be because he's 'ard. Any person who can kick down a building don't care if they get set on fire.
Franky's unlimited supply of cannonballs - does he really have them all stuffed inside himself...
During the Enies Lobby fight I don't know how many times Franky used "weapons left" but it was a lot. His body is simply not big enough to fit all those cannonballs, as well as all that other stuff he's got (chains, cola tank, extra legs, missiles etc).
Possible explanation is that after a couple of shots of weapons left Oda misses out a bit of the fight where Franky runs about regathering his cannonballs for another use. He could have some ultra high tech technology that materialises cannon balls from molecules and particles. Or probably the best explanation would be that this is Oda's One Piece world where things like unlimited cannonballs are a given.
Anyway post any plot holes that you come up with and give an explanation as to how they could be resolved...