cheatin' uh? [fix]

I know there is a thread on this topic and that it’s been closed because it was generating more fuss than solving anything.
I am creating this one as i have discovered something that might lead the developers towards a solution.
Behold this piece of code:

add_action( 'init', 'create_slide_post_type' );
function create_slide_post_type() {
	register_post_type( 'slide',
		array(
                'labels' => array(
				'name' => __( 'Slides' ),
				'singular_name' => __( 'Slide' )
			),
		'public' => true,
                'publicly_queryable' => false,
                'show_ui' => true,
                'show_in_nav_menus' => false,
                'menu_position' => 5,
                'query_var' => false,
                'rewrite' => false,
//                'capabilities' => array('slide', 'slides'),
                'hierarchical' => false,
                'supports' => array('thumbnail', 'excerpt', 'title' ),
                'taxonomies' => array('category')
		)
	);
}

Uncommenting the commented line is triggering the error for me when trying to set a featured image for the custom post.
Feel free to close this thread, but make sure to forward the information to the ones that should see it.
Also if it’s a custom theme that you encounter this error with, you might want to check if the theme is using custom posts (and many do).