/worldborder area is not correct

Do "/worldborder set 50 ~" and try the second command again.

Did it over and over but it is always the same thing http://i.imgur.com/olmGdg9.png

Fail to see a bug here.

If you set the worldborder size to 50 while standing at 0/0 the worldborder spawns from -25/-25 to 25/25.
If you walk to 0/25 and issue /worldborder center ~ ~ or /worldborder center 0 25 the worldborder spawns from -25/0 to 25/50

No there is really a bug because, when i go back to the snapshot 14w17a the border is to the center of myself and when i go back to the snapshot 14w18a/b the border is not to the center, so, i'm right.

downgrading isn't supported, and may corrupt your worlds Nuagedecube

So Snapshot 14w17a was bugged/corrupted ?

downgrading may corupt your world, I didn't say that 14w17a is bugged.
when you go back from 14w18a/b to previous versions (like: 1.7.9 or 14w17a) is downgrading

Yes you are right but why i can not set the border to the center of myself ? it is always as far away to the Nether world (Snapshot 14w18b)

I think the nether border is edited to be that when you build a portal is remains in the border from the overworld

What's the point ? According to your screenshot you're standing on -43/56, issuing the commands /worldborder set 50 and /worldborder center -43 56 spawns the world border as expected from -68/31 to -18/81.