I have a survival world in which I have 4 nether portals: 2 in each dimension.

I have used them for a long time and they have been hooked up correctly all the time (before I updated to 1.8.0).

One time however, I used one and it generated a NEW portal about 100 blocks away from where it was supposed to go (ie: was not supposed to generate anything).

I am sure I did not put a portal there at any time.

After debugging a bit, I think I these are the steps to reproduce:

1. Create 4 nether portals: 2 in each dimension such that each one only goes to its "partner" (Note: I have them too close together ~1100 blocks so that if one was broken, its partner would not generate a new one and instead go to the other (although the wiki says that a new one should be generated if they aren't within 256 blocks? (this doesn't affect the bug though)))
2. In the overworld, go through one into the nether.
3. In the nether, go through the other pair so you are not coming back to where you started.
4. Retrace your steps (go back through the portal you just came out of, go back the the original pair and go through)
5. Search the area for the new nether portal

Note that for some reason, I could not generate the new portal if dawn rose (it was night when I was testing), nor the following night.

Also note that I don't go to the new portal the first time, but I do after it has already been generated.

Notes about images:

1. The 1x4 hole in the center is from the previously generated portal (I was in survival so I mined the obsidian).
2. The portal generated in a different spot this time.
3. You can see the previously-generated portal and the new portal in this image.
4. I managed to get a 3rd portal to generate, but I still can't get any to generate after the dawn (setting the time to night doesn't help).
5. This was taken in the nether where I was standing right by my paired nether portal (the one that is now broken because of the new portal).
6. This was taken in the overworld where I was standing by the new portals.
7. This was taken right at the corresponding position in the overworld of my 1st paired portal in the nether.  On the left are the new portals, on the right is the correct portal (hidden).

Coordinates of overworld portals:

top of 1st paired portal: 30, 67, 26

top of 2nd paired portal: 410, 72, -1013

1st generated portal: 8, 71, -103

2nd generated portal: 12, 71, -94

3rd generated portal: 33, 78, -112

Coordinates of nether portals:

1st paired (now broken) portal in nether coordinates: -3, 106, -5 (in overworld coordinates: -24, 106, -40)
2nd paired portal in nether coordinates: 45, 111, -132 (in overworld coordinates: 360, 111, -1056)