add_filter( 'status_header', 'mf_hack_404', 10 );
function mf_hack_404( $c ) {
	if( defined( 'GALLERY_NAME' ) ) {
		$header = '200';
		$text = get_status_header_desc( $header );
		$protocol = $_SERVER["SERVER_PROTOCOL"];
		if ( 'HTTP/1.1' != $protocol && 'HTTP/1.0' != $protocol )
			$protocol = 'HTTP/1.0';

		return "$protocol $header $text";
	}
	else
		return $c;
}

http://wordpress.org/support/topic/404-on-custom-non-wordpress-pages

Wordpress - 404 problem on physical directories. From the Support Forum.

Wordpress - 404 problem on physical directories

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