The superflat world that I created for 13w41a I loaded up today after installing 13w41b to see if the bug I reported yesterday had been fixed. When the world loaded up I instantly noticed a new bug.

The new bug is that the bedrock fog (the darkening of the sky and all the particle effects), which you will find near y axis 0 (bedrock), is unusually visible on superflats.

I have some pictures to show what I mean.

Im pretty sure this is a bug so to check that it wasnt just a one time thing, I created another superflat in 13w41b and found that the bedrock fog was still really visible. Then I loaded up 1.6.4 and created another superflat and the bedrock fog is not bisible at all, even when you go down 3 blocks to get to the bedrock.
I have some pictures of this too.