Nils Gruendemann

Same problem here, attached log.

[edit]
Ok, for me it only crashes when I click on the "Snooper Settings" button before I load a game. When I'm in a game or after I 'saved and quit' a game, it works as it should. /edit

Same problem here, so attached my crash log as well.
Couldn't submit it via launcher due to my Minecraft being falsely identified as modded.

Same problem, so here my crash report.
Falsely identified as modded here, too.

I can confirm this bug.

Though originally a 14w26b world, I used the MCEdit*uncorrupt-chunks-filter and updated to 26c and then 27b without experiencing any problems. Hadn't been to the nether up to that point either. When I went deeper just now, I nearly suffocated in the netherrack. Couldn't find a portal, though it probably was "somewhere". Dug out to the surface and created a new portal to go back to the overworld, then suffocated in solid blocks. Respawned, went back through my original nether portal and spawned at the portal I had just built in the nether. Took the same one back to the overworld, where I landed in solid blocks again. Dug out in time and dug upwards. Came out to the mountains, where my items were scrambled all over the place - near an unactivated nether portal. Activated it, went through - voila, back at my portal that I built in the nether.