I noticed that my error log was filling up with 404 errors for this asset file that seems to be a part of jQuery UI. I did some digging around and found out that the issue first presented itself when I upgraded from PHP 5.3.10 to PHP 5.3.11 on CentOS 6.5.

Specfically before the update I would see this:

./tmp/assets/asset_manager_global_css.css: background: #fff url(/libs/jquery/themes/base/images/ui-bg_flat_75_ffffff_40x100.png) 50% 50% repeat-x;

Before the update I would see:

./asset_manager_global_css.css: background: #fff url(libs/jquery/themes/base/images/ui-bg_flat_75_ffffff_40x100.png) 50% 50% repeat-x;

Note that the difference is the path changed to absolute which is a bug and incorrect. I'm not sure why a version change would do this but usually this suggests that the code that dynamically generates the path for the install was relying on some side effect or undocumented feature that PHP 5.3.11 deprecated.