I have no exact idea what happened but i went to the nether I had a quest in a book and after 3 sec it just crashed. (It corrupted the save the nether got regened, so I had no portal back and my quest line got reset) I was running 5.0.0beta version. 12.17.0.1987

Crash log: http://pastebin.com/8yJwJ7J7

Just later I found out the Overworld got corrupted as well

Ups... dupe of #211 the last report