I was playing on my new recently created world on the latest snapshot (pre-release 1.6, if im not mistaken that was released 5 July 2013) and I walked a lil bit, than I noticed there was a fail on world generation, which made like a wall of random blocks, like a border. Check the screenshots for you understand what I did mean.

The map is also attached.
You can see the coordinates on the last 3 screenshots.
Thank you.