The animated progress image (a.k.a. horizontal “spinning” dots) that is used to indicate that an asynchronously loaded part of the page is not loading, leaving a broken image indicator. The example I'm seeing most often is in tag dropdowns, but that's not the only location, just the place I see it most. I've only seen this intermittently on SO, and not on any other SE site (including MSO) but I don't visit them all.
This is using Safari 5.0.3 on OSX 10.5.8