Symptoms:
The URL is loadable via normal browsers. However Google Webmaster Tools report crawl error of “not found”. When fetched as Google, it will return “HTTP/1.1 404 Not Found” instead of “HTTP/1.1 200 OK”.

Cause:
I was including WordPress “wp-blog-header.php” in my script such as:

require("subfolder/wp-blog-header.php");

This is the only reason. This is the code that contribute to the error:

if ( !isset($wp_did_header) ) {
$wp_did_header = true;
require_once( dirname(__FILE__) . '/wp-load.php' );
wp();
require_once( ABSPATH . WPINC . '/template-loader.php' );
}

Solution:

Comment 2 lines such as:

//wp();
//require_once( ABSPATH . WPINC . '/template-loader.php' );

I don’t know why, but it cure my crawl error.