Once a page is loaded you can view source and see the error (go ahead and try it on this drupal page). Below is the code being displayed. It is showing php code that should not be visible to the website visitor. I think this is the cached css.
<style type="text/css" media="all">@import "/files/css/f79a1e278e656d085e1470da9a9549b2.css";</style>
Here is the part that is messed up. I think it's from either the common.inc or theme.inc
It should just show this in the source.
<style type="text/css" media="all" href="/files/css/f79a1e278e656d085e1470da9a9549b2.css"></style>
Notice is also needs the href= portion which was missing before.