I was in Hardcore mode on the Local Machine. Started world in 14w26b.
When I went through the portal for the first time, I was spawned in the middle of Netherrack. To avoid suffocation, I knocked out four pieces of Netherrack with my axe. It turned out that the blocks I knocked out were below me; however I did not fall down to the bottom of the vertical tunnel I had created. I was suspended. When I looked around, there was no part of a nether portal evident. In an attempt to find the lost portal, I broke out a block of Netherrack next me and discovered lava on the other side. Game Over!
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Still occurring as of 14w28b, except that other-end portals aren't generating at all. The only way I arrive inside a portal, rather than just in a random piece of air, netherrack or stone, is if I've already built and lit both ends.

@@unknown: Please put any additional findings in the duplicated ticket MC-57524. That is confirmed for 14w28b.
Similar Problem:
Env: Win8 64bit - Java 7.0.25
I created my first portal to go to the nether, walked through and found myself swimming in lava. The portal was created on a one block thick layer of soulsand.
Next time I used the portal to the nether, I arrived in a new portal, which was next to the existing one in a 90 degree angel.
Just a few minutes ago, I build a portal in the nether, to get access to a village, same problem, I got suck inside some wall. The next time I used the portal, there was a new one, right next to it.
The second issue while arriving at the overworld ensures me, there is no issue with a ghast or a creeper, there is a serios bug!