Index: wp-includes/user.php
===================================================================
--- wp-includes/user.php	(revision 40540)
+++ wp-includes/user.php	(working copy)
@@ -2279,12 +2279,14 @@
 	// Check the username
 	if ( $sanitized_user_login == '' ) {
 		$errors->add( 'empty_username', __( '<strong>ERROR</strong>: Please enter a username.' ) );
+	} elseif ( $sanitized_user_login == 'admin' || $sanitized_user_login == 'Admin') {
+		// Don't let somebody fake an admin account
+		$errors->add( 'invalid_username', __( '<strong>ERROR</strong>: Cheatin&#8217; uh?' ) );
 	} elseif ( ! validate_username( $user_login ) ) {
 		$errors->add( 'invalid_username', __( '<strong>ERROR</strong>: This username is invalid because it uses illegal characters. Please enter a valid username.' ) );
 		$sanitized_user_login = '';
 	} elseif ( username_exists( $sanitized_user_login ) ) {
 		$errors->add( 'username_exists', __( '<strong>ERROR</strong>: This username is already registered. Please choose another one.' ) );
-
 	} else {
 		/** This filter is documented in wp-includes/user.php */
 		$illegal_user_logins = array_map( 'strtolower', (array) apply_filters( 'illegal_user_logins', array() ) );