When I create a nether portal, and go through it, it brings me to the nether.
When I travel back from the nether, it occassionally builds a new portal in the main world.
When I go back in to the new portal, it bounces me out and says the portal doesn't have a destination.
The area around the portal becomes glitched and placing blocks doesn't take them out of my inventory.

http://multiverse.github.com/news/better-end-support.html

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