Present in Google Chrome 15, Safari 5.1, Firefox 3.6. Not present in Firefox 7.
Tested on Linux (Ubuntu 11.10) and Windows XP
STEPS TO REPRODUCE
-create 2 overlapping elements e.g. absolutely positioned divs
-apply the same CSS linear-gradient to both of them
-apply a rotate transform to both of them
I've added a jsFiddle link above. If you look closely, you can see a thin dark vertical line to the left of the middle. The effect disappears when you remove the gradient.
No weird dark line.
Confirmed in WebKit Nightly, Version 5.1.2 (6534.52.7, r109280).