Safari 6 rendering issues/artifacts (on Mountain Lion)
I can confirm this is a bug with the Intel Graphics 4000 GPU. I see the graphics artifacts on my 13" Retina MacBook Pro and not on my older MBP which has Mt. Lion and an Nvidia Geforce GPU.

I have first hand experience with this issue while trying to develop css buttons for websites...

I believe the artifacts can be reproduced by using the css3 border-radius in conjunction with a box-shadow. Though obviously it happens in many other circumstances as well. (Which I can confirm)

Here is a link to a bug report on webkit website (though I'm not sure it belongs there):

https://bugs.webkit.org/show_bug.cgi?id=103087

And the SO question it came from:

https://stackoverflow.com/questions/13505314/artifacts-on-css-rendering-box-shadow-on-safari-6-0-2-on-mountain-lion