I noticed that there was hole in the ground on a flat world and saw that there was grass with different colors on top.

I created a flat creative world and built on it for about 30 minutes then logged of and a few minutes later logged back on and noticed about 300 blocks from where I was building there was hole in the ground and colored blocks were around it. I went over to it and saw that the colored blocks were actually grass with different colors instead of the different shades of green on top. On the sides of the blocks it looked like there were snow layers on the grass when there actually wasn't. I took the screenshot seen below and left the world and then I logged back on 5 minutes later and it was gone. I don't know what these mysterious colored grass blocks were. I also noticed a 3x1 hole a few minutes later near where the large hole was. Note that the world I was on was created in 1.1 and had no mods or addons installed.