register_widget is causing 500 errors

I have a multie site setup, and I am trying to activate a theme. When doing so I get a nasty 500 error and the site (including admin dasboard for that site) are down.

Server error
The website encountered an error while retrieving https://localhost/blog.site.com/wp-admin/. It may be down for maintenance or configured incorrectly.
Here are some suggestions:
Reload this webpage later.
HTTP Error 500 (Internal Server Error): An unexpected condition was encountered while the server was attempting to fulfill the request.

I activated the logs and get this error:
[10-Apr-2013 17:46:17 UTC] PHP Fatal error: Class 'Header' not found in /Applications/MAMP/htdocs/blog.site.com/wp-includes/widgets.php on line 324

I traced the header class to my functions.php file for the theme where I am trying to register a widget:

register_widget( 'Header' );
register_widget( 'Search_Widget' );

class Header extends WP_Widget {

	function __construct() {
		$widget_ops = array('description' => __( 'Displays the header.') );
		parent::__construct('header', __('Header'), $widget_ops);
	}

	function widget( $args, $instance ) {
		extract( $args );

		// global $post;
		global $categories, $cat_count, $top_level_cats, $top_level_count;

		$categories = get_the_category($post->ID);
		$cat_count = count($categories);

		/* if the page requested is a post detail or category page */
		if ( is_single() || is_category() ) {
		/* set top level categories based on the nav */
			$top_level_cats = array('brown', 'green', 'blue', 'red');
			$top_level_count = count($top_level_cats);

			for ( $i = 0; $i < $cat_count; $i++ ) {
			/* if we're not in a parent category, then get the parent ID */
			/* DISCLAIMER: we're only going 1 level deep here */
				$slug = $categories[$i]->slug;
				$parentID = $categories[$i]->category_parent;
				if ( $parentID != 0 ) {
					$parent = get_category( $parentID );
				}

				for ( $j = 0; $j < $top_level_count; $j++ ) {
					if ( $parentID == 0 && $slug == $top_level_cats[$j] ) {
					/* if it's a parent category and a top level category */
						$slug = preg_replace("/-/", "_", $slug);
						$banner = get_option( $slug . '_banner');
						$name = $categories[$i]->name;
						break 2;
					} else if ( $parent->category_parent == 0 && $parent->slug == $top_level_cats[$j] ) {
					/* if it's a child of a top level category */
						$slug = preg_replace("/-/", "_", $parent->slug);
						$banner = get_option( $slug . '_banner');
						$name = $parent->name;
						break 2;
					}
				}
			}
		}

		if ( !$banner ) {
		/* if banner was not set by the previous function, set it now */
			$banner = get_option('banner');
			$name = get_bloginfo('name');
		}

		echo "<div class=\"container\">\n";
		echo "<a href=\"" . get_option('home') . "\"><img src=\"" . $banner . "\" alt=\"" . $name . "\"></a>\n";

		wp_nav_menu( array( 'theme_location' => 'blog-menu', 'container' => false, 'menu_id' => 'menu', 'fallback_cb' => '', 'link_before' => '<span class="c">', 'link_after' => '</span><span class="r"></span><span class="b"></span>' ) );

		echo "</div>\n";
	}
}

class Search_Widget extends WP_Widget {

	function __construct() {
		$widget_ops = array('description' => __( 'New search widget for the right rail.') );
		parent::__construct('search_widget', __('Search Widget'), $widget_ops);
	}

	function widget( $args, $instance ) {
		extract( $args );

		echo "<div class=\"pod wp-search\"><div class=\"hd\"></div><div class=\"bd\">\n";
		include('searchform.php');
		echo $after_widget;

	}
}

I am completely baffled.