wasn't really sure how to put it into words so i've uploaded a short video on youtube with what you see. It seems to have something to do with the render distance (or maybe that's to do with it updating). I started the game on render distance 10 so i changed it to 16 which is what i play on, and then it happened.

http://www.youtube.com/watch?v=f5PA7-T8EoE