By default, when you (or someone else) fail to login on your wp-admin, WordPress displays an error message. While those messages might be useful, they can also be used by hackers to track their progress. Here is a nice tip to prevent WordPress from displaying login errors.
Paste the following line of code into your functions.php file:
add_filter('login_errors', create_function('$a', "return null;"));
Once you saved the file, WordPress will not display any login error messages anymore.
Thanks to WP Tuts for the tip!