I ran across an odd behaviour today: I had the Pixopoint menu plugin 0.6.5 unpacked in a directory outside my WordPress installation, and symlinked into the wp-content/plugins directory.
Symptoms of the failure: nothing seemed to be happening. I could tell the plugin was installed because I had the correct contents for the menu, but the CSS was mine (even with the option to use the plugin’s CSS) and there were no flyout menus, the list was fully expanded all the time.
Removing the symlink, and putting the directory directly in the wp-content/plugins directory, cured the problem, and everything’s working just fine again.
The only clue I could find was by viewing the CSS manually, where the file that should have included the plugin instead had a PHP error from the first line of code (line 18) in css/style.php:
require_once("../../../../wp-load.php");
saying that it couldn’t find the file via that path.
This is just FYI really; the wordaround of not using the symlink worked for me, and I don’t know enough about WordPress to suggest a source-level fix.