Tony Edwards spotted a small and subtle test by Google and posted it on Twitter. Google is testing placing a bolder/darker line break line between the ads and the organic results in the search results.

Here is an animated GIF with his screen capture and what most users see:

The one most people see is the faded, hard to see, line break between the ad unit and the organic results. The one Google is testing is the bolder and darker line break line between the ad unit and the organic results.

Clearly, the bolder separator is easier to understand, which might be what Google is not aiming for.

Forum discussion at Twitter.