I had a strange problem today with Expression Engine, in that the CSS style sheet was not being displayed properly in the browser.  All of content od the page would appear, and the style sheet was being linked in, but the contents of the style sheet was not being applied.  After a little investigation I discover that is was a problem with the path.php file, which I had recently amended.

The problem was a space after the closing quotes of path.php.  If you have the same problem ensure your path.php file does not have a space after the  ?> that closed the file.