This is supposed to happen...
This world was not reused.
I don't get it, that's a different error.
They should undo the stupid 30,000,000 block boarder thing and then this problem would be solved. Anyway Minecraft is supposed to have infinite worlds, right? _
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Seems kind of odd. Even if you was intended, they would've changed it by now due to the shear number of votes, or else, closed the issue.
Please add that it affects Mac OSX.
Except that it was never fixed...
I'm not sure this is fixable from a developers' standpoint.
The but occurs by deleting a world and making a new one that is named the same thing (as the desc says).
It has to do with the way computers delete things. When a file is deleted, it isn't actually deleted, it's just allowed to be overwritten. So if you create a new world right AFTER deleting one of this same name, it will create the world in the same place on the hard drive, and thus it will pull some chunks from the deleted world.
So that's why it isn't fixable- at least for the developers.
There are workarounds (like 'permanent' deletion), but they are complicated.