Hi,
I just found a conflict between the And the Winner Is ... and Members plugins (http://wordpress.org/extend/plugins/members/). Specifically if AtWI is activated, you can't add new capabilities for users, and the existing capabilities are truncated. According to the Members plugin author Justin Tadlock in his support forum:
"Yep, it looks like that plugin is inappropriately loading JavaScript on every page in the WordPress admin instead of only the plugin-specific admin pages.
If you want to pass a note along to the plugin author, tell him that his JavaScript should not be hooked to admin_init as in the atwi.php file:
add_action('admin_init', array(&$this, 'AdminInit'));
It should be hooked to admin_enqueue_scripts and the $hook_suffix parameter should be used to determine which page is currently being viewed.
Or, just tell him to shoot me an email."
Greek to me, but I'm passing the word along!