I have a world I created back in 1.7.2. It's been smooth transitions through the snapshots, and luckily i missed 14w26a/b.

This issue happens when I use portals created months ago in a previous version. The overworld side is fine, but on the nether side I am spawned in a new portal ~10 blocks from the old one. The old one is still there but not lit. Then I travel about 800 blocks through the nether to reach the portal that goes to my other base, and it is not lit either. I didn't have any flint and steel on me so I couldn't try just lighting it. I ran back to the working portal and back to the overworld and took the long way to my other base. That portal was still lit, and when I jumped to the nether, same thing, a new portal was created about 10 blocks away and the old one was not lit.

One little detail ... I like my portals boxed in so they are only open to one side. ie I build a wall on one side so I cant walk straight through but enter and exit from the same side. maybe this is part of the problem since the new portals are created where they are open to both sides. It was never a problem before to have one side walled off.

The only change I made is updating the snapshot - I haven't even been to the nether in months (until this recent trip), let alone messed with any portals.