New as of 14w28a: The first portal you create in a world has a small chance of spawning you underneath the bedrock of the nether in the void, spawning you in a wall, and generating the portal unlit. After this, any newly created portal in the overworld automatically links to the original portal in the nether, no matter how far away.
Updated steps to reproduce:
1. Create a nether portal in the overworld and travel through it, preferably in creative due to the chance of spawning in the void, spawning in lava or suffocating to death. Remember your surroundings when you enter the nether.
2. Re-enter the overworld and create another portal any distance away from the original and travel through it. You should be in the exact same place as before.