Index: wp-includes/user.php
===================================================================
--- wp-includes/user.php	(revision 45005)
+++ wp-includes/user.php	(working copy)
@@ -2446,7 +2446,7 @@
 		/** This filter is documented in wp-includes/user.php */
 		$illegal_user_logins = array_map( 'strtolower', (array) apply_filters( 'illegal_user_logins', array() ) );
 		if ( in_array( strtolower( $sanitized_user_login ), $illegal_user_logins ) ) {
-			$errors->add( 'invalid_username', __( '<strong>ERROR</strong>: Sorry, that username is not allowed.' ) );
+			$errors->add( 'invalid_username', __( 'Sorry, that username is not allowed.' ) );
 		}
 	}